If you're just joining us we're covering the global response after North Korea conducted its sixth nuclear test, a provocative breach of sanctions and a major escalation of the rumbling crisis.

Here's what we know so far:

• An artificial earthquake was detected early on Sunday in North Korea, a clear sign of an underground nuclear test.
• A second, smaller tremor was detected shortly afterwards, which may have been structures at the test site collapsing.
• South Korea said the tremor was nearly 10 times more powerful than that caused by the last nuclear test.
• North Korea has issued a statement claiming to have carried out a successful test of a hydrogen bomb, the most powerful weapon ever created.
• The device had an estimated yield of 100 kilotons, that five times more than the bomb dropped on Nagasaki.
• It has also released pictures purporting to show a hydrogen bomb being placed on an intercontinental ballistic missile, the last stage is becoming a genuine nuclear armed state.
• Japan and South Korea have condemned the test.

There is an international treaty banning all nuclear weapons testing. But eight nuclear countries have not signed up to the Comprehensive Nuclear-Test-Ban Treaty (CTBT) so it is not yet in force. That includes North Korea.

“If confirmed as a nuclear test, this act would indicate that the DPRK's nuclear programme is advancing rapidly," Lassina Zerbo, executive secretary of the Comprehensive Nuclear-Test-Ban Treaty Organization, has said in a statement, external.

DPRK (Democratic People's Republic of Korea) is North Korea's official name.

Prof Daniel Pinkston from Troy University in Seoul says Pyongyang's sixth nuclear test did not come as a surprise:

South Korea says its top security officials have been in contact with the US to discuss what next steps to take.

The Yonhap news agency said National Security Office chief Chung Eui-yong had a 20 minute phone call with his US counterpart, HR McMaster, not long after the North issued its claim.

This is the full statement from North Korea on its hydrogen bomb test.

Image source, Reuters

The North Korea TV announcement about the nuclear test was delivered by the country's - and perhaps the world's - most famous news anchor, Ri Chun-hee.

She has cried, laughed and shouted on Korean Central Television for over 40 years.

What we know about Ri Chun-hee, the most famous woman in North Korea

This is the first nuclear detonation of Donald Trump's presidency. He hasn't yet commented - in recent hours he's been tweeting about the US flood disaster.

He has previously threatened to unleash "fire and fury like the world has never seen" if North Korea continued to threaten the US and that years of diplomacy have achieved nothing.

It's reasonable to assume his response to this development will not be a call for further talks.

Image source, AFP

The North Korean crisis is complex and has deep roots. And the actions of the North Korea leadership are often hard to understand.

If you're looking for a very basic primer on the issue to help you understand today's developments, try this: North Korea crisis in 300 words.

The BBC's Tessa Wong is monitoring reaction on Chinese social media. She says some people close to the North Korean border are expressing alarm about the latest test.

One commenter on Caixin says: "Why are we not admitting that the threat North Korea poses to China already far, far exceeds that posed of the US." Another calls on China, the North's only real ally, to "immediately cut relations with North Korea!! Cut North Korea's oil!! Close all trade links with North Korea".

North Korean TV has broadcast images appearing to show leader Kim Jong-un signing the order to carry out the nuclear test.

Image source, North Korea TV

Image source, North Korea TV

Image source, Reuters

North Korean state TV says its hydrogen bomb can be loaded onto a missile, the AFP news agency reports.

North Korea says it has carried out a hydrogen bomb test - calling it "a complete success".

This news has not come out of the blue - North Korea has been threatening for some time to carry out its sixth nuclear test. The isolated and paranoid state has long insisted that it has to have a viable nuclear weapon to prevent its enemies from wiping it out.

But as John Nilsson-Wright of Chatham House writes: "Mr Kim's nuclear and missile testing ambitions are also an expression of identity politics. The legitimacy of the Kim dynasty's political leadership is rooted in a narrative of defence against an implacably hostile United States."

What does North Korea really want?

Image source, Reuters

In just over a decade, North Korea has been able to increase the power of its nuclear explosions from 4.3 magnitude in 2006 to 6.3 magnitude today.

South Korean Finance Minister Kim Dong-yeon will convene an emergency staff meeting later to assess the impact of the suspected North Korean nuclear test on the local financial market, according to the Yonhap News Agency.

Immediate estimates of the size of the detonation on Sunday - based on the size of the quake it generate - have put it at 100 kilotons.

In comparison, the bomb dropped by the US on Nagasaki in August 1945 was about 20 kilotons.

The Nagasaki bomb killed more than 70,000 people instantly.

Image source, Getty Images

South Korea's defence committee says the force of the blast was about 100 kilotons.

Vipin Narang, associate professor of political science at the Massachusetts Institute of Technology, describes this as a "huge milestone for deterrence purposes" because it has the force to destroy a city.

Japan's Chief Cabinet Secretary Yoshihide Suga said sanctions options against North Korea - which is already under wide ranging restrictions - include new limits on oil-products trade.

South Korean nuclear security expert Duyeon Kim tells the BBC that South Koreans are sceptical the US would act if the north attacked it.

"If Washington feels that its homeland is at stake, would Washington sacrifice South Korea to save its own homeland?" she says.

"That's the biggest question on most South Koreans' minds."