US President Joe Biden says he has spoken with his Ukrainian counterpart, Volodymyr Zelensky, and briefed him on the steps he is taking "to rally international condemnation".

In a statement, Biden says he condemned the attack as "unprovoked and unjustified".

Zelensky "asked me to call on the leaders of the world to speak out clearly against President Putin's flagrant aggression, and to stand with the people of Ukraine", Biden adds.

The US president says he will be meeting with G7 leaders on Thursday, and the US and allies "will be imposing severe sanctions on Russia".

"We will continue to provide support and assistance to Ukraine and the Ukrainian people," he says.

Image source, Getty Images

BBC Kyiv correspondent James Waterhouse says a Ukrainian government official has given an indication of the breadth of the Russian military action so far.

The official says there have been cruise missile strikes launched this morning on Kyiv, as well as troop movements in Odessa to the south of the country.

Troops have also been crossing the border in Kharkiv, about 25 miles from the Russian border, the official says.

Early signs according to witnesses and government officials are that this is something quite large-scale, Waterhouse says.

There are also multiple reports from local media citing Ukraine's Interior Ministry which says some of the missile strikes have attacked Ukraine's military missile command centres and military headquarters in Kyiv.

Russia's defence ministry has denied attacking Ukrainian cities - saying it is targeting military infrastructure, air defence and air forces with "high-precision weapons".

US Senator Mark Warner, the chairman of the Senate Select Committee on Intelligence said it is time to "up the pain level" on Russia following the beginning of its military operation in Ukraine.

In a statement, Warner said that US lawmakers from both sides of the political spectrum should come together - along with US allies - to "demonstrate to Putin that this aggression will not be allowed to go unpunished".

Warner added that Russia's operation in Ukraine is a tragedy for the Russian and Ukrainian people alike.

"They [Russians] will pay a steep cost for Putin's reckless ambition, in blood and in economic harm," he added.

Russia's defence ministry has denied attacking Ukrainian cities - saying it is targeting military infrastructure, air defence and air forces with "high-precision weapons", the country's state agency RIA citied the ministry as saying.

Prime Minister Boris Johnson says he is "appalled by the horrific events in Ukraine" and that Russian President Vladimir Putin "has chosen a path of bloodshed and destruction by launching this unprovoked attack".

He adds that he has spoken to Ukraine's president to discuss how to respond and promises decisive action by the UK and allies.

Sarah Rainsford
BBC Eastern Europe Correspondent

Kramatrosk, Donetsk region: I was woken shortly before 5am by a loud thud.

I was barely asleep anyway, still dressed, after Ukraine President Zelensky’s passionate dawn appeal directly to the Russian people to stop this war.

It sounded even then like a last-ditch effort; hopeless.

Rushing to pack, I watched Vladimir Putin in Moscow, snarling about a supposed "genocide" here in eastern Ukraine - which is a flat lie - and the need for "denazification" in Ukraine. It was surreal.

So was the chilling threat to the West not to intervene: Putin had already reminded us that he has nuclear weapons.

Only last night in Kramatorsk I was with crowds of hundreds who were on the main square, wrapped in Ukrainian flags and mounting a small act of defiance, singing the anthem and patriotic songs. There were lots of children.

Last night, the restaurants were busy. Flower shops open. Cars on the streets. Everyone I spoke to was scared. No one knew what to expect.

But I don’t think they expected this full-on assault on their country.

Now people I’ve spoken to are frightened, it’s like they’re frozen. No one even knows where to run to, if they wanted.

Ukraine's President Zelensky has confirmed reports of missile strikes in the country, according to a Reuters report.

He says Russia has carried out missile strikes on Ukraine's infrastructure and on border guards.

Ukraine's President Volodymyr Zelensky is calling on his national security and defence council to declare martial law.

The council is expected to hold an urgent meeting to decide the issue.

Oil prices jumped above $100 for the first time in more than seven years as President Putin announced a "military operation" in eastern Ukraine.

News also caused Asian stock markets to trade lower by 2 to 3%. Markets have been taking a battering over the last few days as tensions escalated - prices hit $98 a few days ago when Mr Putin tore up a peace deal and ordered troops into two rebel-held eastern regions.

The BBC's Asia business correspondent Mariko Oi says investors have been fleeing to what they consider safe-haven assets. Gold prices are at the highest level in over a year while the US dollar and the Japanese yen are also strengthening.

Liz Truss, the UK foreign secretary, has tweeted, saying she strongly condemns "the appalling, unprovoked attack President Putin has launched on the people of Ukraine".

She says the UK stands with Ukraine and will make a joint response with international partners to "this terrible act of aggression".

Paul Adams
BBC diplomatic correspondent

The first explosions were heard here in Kyiv shortly after five in the morning. There were four or five, muffled and distant. Others followed soon afterwards, the most recent sounded closer but still not in the city centre.

One report suggested the airport might be under attack. A government official dealing with the foreign press said airfields and military headquarters had been attacked.

Other reports have been coming in from across Ukraine, including towns in the east, like Kramatorsk, close to areas controlled by pro-Russian separatists. Coming shortly after Vladimir Putin’s latest speech, this appears to mark the start of Russia’s long-awaited attack.

Nato Secretary-General Jens Stoltenberg has condemned Russia's "reckless attack" on Ukraine.

In a post on Twitter, he says it "puts at risk countless civilian lives". "This is a grave breach of international law & a serious threat to Euro-Atlantic security," he adds.

He says Nato allies will meet to address Russia's "renewed aggression".

Former US President Donald Trump took to the airwaves on Fox News as the Russian offensive began on Wednesday night US time to argue that it "wouldn't have taken place" during his administration.

Trump phoned in to say he didn't believe that Putin "wanted to do this, initially".

"I think he wanted to do something and negotiate, and it got worse and worse, and then he saw the weakness," Trump said.

Additionally, Trump said that he believes the Russian invasion was partly prompted by the "weakness" of the US withdrawal from Afghanistan.

Ukraine's foreign minister has tweeted: "Putin has just launched a full-scale invasion of Ukraine."

He's stressed that Ukraine "will defend itself" and that "the world can and must stop Putin".

Ukraine's UN ambassador Sergiy Kyslytsya says Russia's UN envoy has confirmed that his president declared "a war on my country", reports Reuters news agency.

The Ukrainian interior ministry tells CNN: "The invasion has begun."

In his televised address to the Russian people on Thursday morning, Russian President Vladimir Putin said his country has "no plans" to occupy Ukraine.

In his remarks announcing a "special military operation", Putin said that the Ukrainian people would be able "to choose freely" who runs the country.

News agency Reuters is quoting a witness as having heard an explosion in the Belgorod province in south-west Russia, which is not far from the Ukraine border.

It is unclear what the explosion could be, but CNN reporter Frederik Pleitgen in Belgorod is saying he has heard outgoing artillery fire, as well as rocket fire and jets that may be flying towards Ukraine.

US President Joe Biden has responded to what he calls an "unprovoked and unjustified attack by Russian military forces".

He says "the prayers of the entire world are with the people of Ukraine".

"President Putin has chosen a premeditated war that will bring a catastrophic loss of life and human suffering," he says.

"Russia alone is responsible for the death and destruction this attack will bring, and the United States and its allies and partners will respond in a united and decisive way. The world will hold Russia accountable."

Biden says he will address Americans on Thursday about the consequences Russia will face.

He says he will be monitoring the situation from the White House and will meet with G7 leaders in the morning before announcing "further consequences" for Russia.

Biden says there will be a "strong, united response" from Nato allies that deters any aggression against its members.

"Tonight, Jill and I are praying for the brave and proud people of Ukraine," he says.

We are hearing of reports - some unconfirmed - of explosions in other parts of the Ukraine.

BBC correspondents earlier confirmed they heard loud bangs in the capital Kyiv, as well as Kramatorsk in the Donetsk region.

But there have also been reports on social media of explosions in Kharkiv in the north-east, Odesa in the south, and the eastern Donetsk Oblast area.

The BBC's Paul Adams in Kyiv has just reported hearing five to six "distant explosions" in the Ukrainian capital, and says there are reports are coming in of explosions elsewhere in the country.

Local media report that gunfire was heard on Thursday near the main airport of Boryspil in Kyiv.