As night falls in the US, here are the latest key developments from there:

• Secretary of State Antony Blinken held a call with the foreign ministers of G7 countries, as well as Ukraine, in which he said the group was "supporting the Ukrainian people and imposing severe costs and consequences to hold Russia accountable for its war of choice"
• The State Department issued a warning to American citizens currently in Russia, advising them to leave "immediately"

• The governor of New York signed a bill banning the state government from doing business with Russian entities
• New York City's famed Metropolitan Opera announced that it was cutting ties to pro-Putin musicians and artists
• Republican 2012 presidential candidate and current senator Mitt Romney rebuked members of his party who have voiced support for Russia, calling them "morons"
• Anti-Russia protests have continued across the US
• In Chicago's Ukrainian Village neighbourhood, Governor JB Pritzker used an expletive to denounce Putin to the crowd

Image source, Getty Images

Katya Adler
Europe Editor

The EU relies heavily on Russia for energy. Moscow makes a lot of money as a result. And those two facts can’t be separated from the current crisis.

Brussels has imposed more and more sanctions on the Kremlin after its invasion of Ukraine. At the weekend EU leaders announced a raft of new sanctions - targeting Russia’s central bank, closing EU skies to Russian aircraft, and buying and sending weapons to help Ukraine. But - with the exception of Germany suspending approval of a gas pipeline to Russia - the EU has not cut energy imports.

Energy prices in Europe are already high. Scarcer supplies could send them skyrocketing - as Vladimir Putin knows. Brussels now worries Moscow may retaliate, reducing or halting energy exports.

Weaning off this reliance on Russia and ensuring energy security by other means are big priorities on EU minds.

Energy ministers from the EU are holding an emergency meeting in Brussels on Monday amid concerns that energy supplies could be disrupted by the conflict in Ukraine.

The EU relies on Russia for 40% of its gas and more than a quarter of its oil.

Now it’s concerned Vladimir Putin could block deliveries after the EU announced new sanctions against Russia this weekend.

Would Putin push the nuclear button?published at 02:24 Greenwich Mean Time 28 February 2022

02:24 GMT 28 February 2022

Steve Rosenberg
BBC Moscow Correspondent

Image source, Getty Images

So many times, I’ve thought: “Putin would never do this.” Then he goes and does it.

“He’d never annex Crimea, surely?” He did.

“He’d never start a war in the Donbas.” He did.

“He’d never launch a full-scale invasion of Ukraine.” He has.

The phrase “would never do” doesn’t seem to apply to Vladimir Putin.

And that raises an uncomfortable question: “He’d never press the nuclear button first.... would he?”

It’s not a theoretical question. Russia’s leader has just put his country’s nuclear forces on "special" alert, complaining of “aggressive statements” over Ukraine by Nato leaders.

When he announced on TV his “special military operation”, he delivered a chilling warning: “To anyone who would consider interfering from the outside - if you do, you will face consequences greater than any you have faced in history.”

“Putin’s words sound like a direct threat of nuclear war,” says Nobel Peace Prize laureate Dmitry Muratov, chief editor of the Novaya Gazeta newspaper.

"This is a threat that if Russia isn’t treated as he wants, then everything will be destroyed”.

If Putin did choose a nuclear option, would anyone in his close circle try to dissuade him? Or stop him?

“Russia’s political elites are never with the people,” says Muratov. “They always take the side of the ruler.”

And in Putin’s Russia the ruler is all-powerful.

Read Steve's article in full

One refugee at the Polish border breaks down as she tells the BBC's Mark Lowen why she fled Ukraine.

"When kids are crying, when my child was scared to take off her socks at night because of being so scared, you don't know what to do with yourself," she says.

Her family remains in Chernihiv, a northern city currently under assault by Russian forces. They sit in the cellar each day, she says.

So far, some 368,000 Ukrainians have fled their homes since Russia's invasion last week, according to the United Nations High Commissioner for Refugees.

Anthony Zurcher
BBC North America reporter

The Conservative Political Action Conference, or Cpac for short, is usually a good way to gauge the mood of grass-roots activists within the Republican Party in the US.

If the four-day event that concluded in Orlando, Florida, on Sunday night is any indication, the conservative base of the party is much more interested in the upcoming congressional mid-term elections in November and defeating Democrats than they are about Russia’s invasion of Ukraine.

Top Republicans in office like Florida’s Governor Ron DeSantis, Texas Senator Ted Cruz and Florida Senator Rick Scott, didn’t mention Ukraine at all in their speeches.

Donald Trump, who took the stage on Saturday night, offered a quick condemnation of the invasion, but spent more time defending himself.

First over his interactions with Ukrainian President Volodymyr Zelensky that led to his first impeachment.

And then for his statements earlier this week calling Russian President Vladimir Putin a “genius”. He added that Nato nations and current US leadership were "dumb".

One of Trump more loyal supporters, Florida Congressman Matt Gaetz, went further, questioning why the US should be “footing the bill for Ukraine” by imposing sanctions that could damage the American economy.

BBC Ukrainian

Image source, Getty Images

Over 90% of Ukrainians surveyed in a poll this weekend said they support President Zelensky - a threefold increase from December last year.

According to the poll, conducted by the respected Rating Sociological group, 91% backed him, only 6% do not support him and 3% were undecided.

Two thousands respondents from across Ukraine were questioned, though residents in the Crimea and the rebel-held areas in eastern Ukraine were excluded.

When asked about the chances of Ukraine being able to repel the Russian assault, 70% said they believed it was possible.

Support for the Ukrainian armed forces has also increased considerably since just before the start of the invasion.

Read more about the comedian who became president

Michelle Fleury
BBC business reporter, New York

Image source, Getty Images

In the first day of trading since harsh new sanctions were imposed, the Russian rouble plunged nearly 30%, to a new record low against the US dollar. The euro sank more than 1%, while the price of oil surged.

The measures introduced this weekend increase the financial and social costs of Russia’s invasion of Ukraine.

Russians are already waiting in long queues, worried that their bank cards may stop working or that limits will be placed on the amount of cash they can withdraw.

And according to regulators, some of the European operations of Sberbank, the Russian state owned bank, are failing.

The new ban on the Central Bank of Russia’s ability to use its roughly $630bn (£471bn) in foreign reserves undermines its ability to defend the rouble. Inflation is likely to go up because of the currency’s weakness.

This leaves the central bank with a few options, including raising interest rates or limiting the amount of money that can be brought into or out of the country.

Images released by BlackSky, a global satellite monitoring company, show craters that resulted from shelling in the northeastern city of Kharkiv.

Ukrainian authorities said Russian forces had blown up a natural gas pipeline in the city, Ukraine's second largest, in the early hours on Sunday. Craters from artillery fire can be seen near residential areas in the satellite image taken on Sunday afternoon.

Ukrainian authorities said they regained control of the city despite the Russian assault.

Image source, BlackSky/Handout via Reuters

BBC Russian

The Russian communications watchdog Roskomnadzor has blocked “Ishchi svoikh” ("Look for your own"), a website created by Ukrainian authorities for the families of Russian soldiers who had been killed or taken prisoner in the Russian invasion of Ukraine. The website publishes photographs or documents of the men.

The Ukrainian Interior Ministry had announced on Saturday it was setting up the website and a social media channel with information about the Russian casualties and prisoners of war (POWs) to make it possible for the relatives in Russia to find out what happened to their loved ones.

Ukrainian presidential administration adviser Oleksiy Arestovych said the website was “our gesture of goodwill towards Russian mothers”.

Russian authorities admitted for the first time on Sunday that there had been Russian casualties in the course of the fighting that began on 24 February, but did not give any figures.

Ukraine says that 4,300 Russian soldiers have been killed since the start of the war. These numbers have not been verified.

Image source, Getty Images

A vote in Belarus has cleared the way for the former Soviet state and current Kremlin ally to get nuclear weapons for the first time since the country gained independence in 1990.

A referendum taken on Sunday approved plans for Belarus to adopt a new constitution that takes out its current non-nuclear status.

The move has serious implications for proliferation, and potentially makes Belarus a staging zone for the Russian assault on Ukraine.

Belarusian leader Alexander Lukashenko tightly controls the country and has closely aligned himself with Putin.

Speaking at a polling station on Sunday, Lukashenko said: "If you [the West] transfer nuclear weapons to Poland or Lithuania, to our borders, then I will turn to Putin to return the nuclear weapons that I gave away without any conditions."

The referendum also cements Lukashenko's time in office until 2035. He has been in power since 1994.

Between 1994 and 1996, former Soviet states Belarus, Ukraine and Kazakhstan all agreed to give up nuclear weapons in exchange for security assurances - assurances that have now been broken.

In a phone call with UK Prime Minister Boris Johnson this evening, Ukraine's President Volodymyr Zelensky said he believes the next 24 hours will be a crucial period for Ukraine as fighting continues across the country.

During the call, Mr Johnson praised Mr Zelensky's leadership since Russia's invasion, according to a UK government spokesperson.

Mr Johnson added the UK would do all it can to make sure defensive aid from the UK and its allies reaches Ukraine.

The two leaders agreed to stay in close contact.

Jonathan Marcus
Defence analyst, Exeter University, UK

Image source, Getty Images

Military success for Russia has so far been limited. This was a half-baked and poorly executed operation. The latest fighting in Kharkiv repeated earlier mistakes, with lightly armed forces entering the city and heavier weaponry deployed in small packets.

Mr Putin chose speed over preparation. He did not secure full air superiority. His advancing armoured units did not secure their lines of communication, leaving the logistics convoys vulnerable to attack. Russian soldiers when confronted by ordinary Ukrainians seem confused and uncertain.

Mr Putin’s invasion met an unexpected level of resistance. His Ukrainian opposite number galvanised his people and spread Ukraine’s message far and wide.

The invasion has prompted a seismic change in German foreign and defence policy along with that of the EU. Arms supplies to Ukraine will continue to flow. Finland and Sweden may end up even closer to Nato.

Image source, Getty Images

Canada's federal government will send an additional C$25m ($19.6m; £14.7m) in additional non-lethal military aid to Ukraine.

Deliveries will include helmets, body armour, gas masks and night vision gear. It follows an initial C$7m delivery of weapons.

Announcing the commitment on Sunday, Canada's Defence Minister Anita Anand said deploying Canadian troops on a combat mission to Ukraine was "not on the table" for Canada or Nato allies.

Last week, Anand said Canada had 3,400 Canadian Armed Forces personnel on standby to join Nato's main response force to defend territories within the alliance. Ukraine is not a Nato member.

As in many parts of the world, demonstrations in support of Ukraine were held across Canada on Sunday.

With some 1.3 million Ukrainian-Canadians, Canada has the third-largest Ukrainian population in the world, behind Ukraine itself and Russia.

After a weekend of fierce fighting, the assault on Ukraine from Russia has continued past a fourth day.

For readers just joining us, here's what happened on Sunday:

• Intense fighting continued through the weekend, with Russian forces targeting cities in Ukraine, but ultimately failing to hold the key city of Kharkiv, in the northeast of the country, and its second largest
• However, the assault from Russia continues from almost all directions, and Kremlin forces were said to be making better progress from the south, besieging the port cities of Kherson and Berdyansk, on two parts of the Black Sea
• Ukrainian President Volodymyr Zelensky announced plans to meet a Russian delegation on the Ukrainian-Belarusian border after talks with Belarusian leader Alexander Lukashenko. No timeline was offered and the situation is vexed, given Belarusian support for Putin's invasion
• Earlier, the Ukrainian government said it had petitioned the International Court of Justice to bring charges against Russia for its invasion
• European and US allies accused the Kremlin of ratcheting up tensions further after President Putin put Russian nuclear defences on "special alert"
• Residents in the capital city of Kyiv remain under curfew as it braces for further assault, with reports of more Russian troops headed to the city
• Russians are braced for financial chaos ahead of markets opening on Monday after tough sanctions were announced over the weekend that would freeze Russian banks out of the international financial system

Siobhan Leahy
BBC World Service

Image source, Getty Images

More now on the EU agreeing to take in Ukrainian refugees for up to three years without asking them to first apply for asylum.

The BBC has been told that the vast majority of European interior ministers are in favour of invoking the EU’s Temporary Protection Directive for the first time ever, to cope with the mass influx of displaced Ukrainians arriving in EU states.

Currently, Ukrainians can stay visa-free within the EU for 90 days. The EU says invoking the temporary protection directive would make it easier for Ukrainians to temporarily settle in EU countries, removing all paperwork associated with typical asylum applications.

Having received a majority of support, ministers can now take this to the next council meeting on Thursday. The commission will then have to come up with a full proposal for member states to sign up to.

Image source, Getty Images

The Russian rouble has nose-dived nearly 20% - plunging to a new record low versus the US dollar. The fall in value comes after Western nations announced a severe set of sanctions following the invasion of Ukraine.

Russia's attack - and subsequent sanctions - have wrought havoc on markets around the globe. The euro sank around 1%, as did the Australian and New Zealand dollar.

Katya Adler
Europe Editor

The violence on the ground in Ukraine and the fact that the EU sees Vladimir Putin's actions as an attack on wider European stability and security has focused minds in the EU.

It's working in a much faster, more united way than we normally ever see.

The EU today announced for the first time that it would be purchasing weapons and delivering them to a country in conflict, Ukraine.

The words "unprecedented" and "watershed moment" are something you're hearing a lot in Brussels at the moment, but it's actually relevant.

Over the weekend, Germany really made the headlines across all of Europe by turning its modern day defence policy completely on its head.

It is traditionally very wary of getting involved in military conflict because of its history in World War Two.

Now it's saying that it will be sending weapons directly to Ukraine and it will be massively investing in its own military, which will come as a relief to Nato allies.

But one of the concerns in Brussels tonight is that because of this coordination in the West and because of the level of the sanctions imposed, if Vladimir Putin now feels on the back foot, what could he do next?

The EU is sending arms to Ukraine and banning Russian flights. Here's what you need to know after Day Four of the war.

Ukraine's military has just given an update on Sunday's fighting.

Writing on Facebook, external, the general staff of Ukraine's armed forces described Sunday as "a difficult time" for the military, saying Russian troops "continue shelling in almost all directions".

Latest reports suggest Russian troops have taken Berdyansk, a southern port, but earlier Ukraine said it had fought back an attack on the country's second biggest city, Kharkiv.