A massive nuclear test like this causes global alarm, but it does not mean that a nuclear device is likely to be deployed in anger any time soon.

The widely held view is that it would be suicidal for North Korea to carry out a nuclear attack - they would almost certainly be eradicated within minutes in retaliation. As one expert in Seoul told the Guardian newspaper last month: "They want to get the ability to wipe out Chicago from the map first, and then they will be interested in diplomatic solutions."

How worried should you be?

China's Nuclear Safety Administration says it has begun monitoring for radiation along the border after North Korea's claimed hydrogen bomb test.

If you're the sort of person that likes a list, South Korea's Yonhap news agency has put together this chronology of all the major developments, external from North Korea's first nuclear test in 2006 to its sixth and most powerful in the last few hours.

You'll recall that Guam has been the most recent focus of North Korea's threats. Pyongyang said in recent weeks it was preparing to fire missiles towards the US Pacific territory, with its 160,000 US citizens and US military bases.

Guam's leadership has been doing its best to keep people there calm, and issued a statement in the last few hours telling people there was no immediate threat to the island or the wider Marianas area from the nuclear test.

Chung Eui-yong, a key advisor to South Korea's President Moon Jae-in, says they will press for the most powerful sanctions by the UN Security Council to completely isolate the communist state, according to the Yonhap News Agency.

Here is North Korean newsreader Ri Chun-hee announcing the nuclear test, in her usual dramatic tone and with her usual traditional outfit.

She called it "a very meaningful step in completing the national nuclear weapons programme".

The nuclear test will likely overshadow the BRICS summit, taking place in Xiamen, China. That's a gathering of the leaders from the emerging economies of Brazil, Russia, India, China and South Africa.

Russia and China have both been the most reluctant voices when it comes to penalising North Korea for its nuclear ambitions. Our correspondent watching the summit, Stephen McDonell, says North Korea has not yet been mentioned in Chinese President Xi Jinping's opening speech.

South Korea is considering its response and says one option is to deploy the "most powerful US tactical weapons" - widely seen as a reference to US nuclear weapons.

The tremor from the powerful nuclear blast was felt across the border in China. Some people said they ran out of their homes in fear.

One person wrote on Chinese microblog Weibo: "I put my underpants on and I just ran, and when I reached the first floor I can say I wasn't the only one running away with just my underpants on!"

"I was lying down and sleeping when the tremor woke me up. At first, I thought it was a dream," said another, as reported by AFP.

China's Ministry of Foreign Affairs has just released a statement saying it "resolutely opposes" and "strongly condemns" the nuclear test. You can read the statement here in Chinese., external

Here's more from Japan's Foreign Minister Taro Kono: "The Japanese government has condemned the North in the strongest terms through its embassy in Beijing, saying that if the country had in fact conducted a nuclear test, it is unforgivable and violates the UN Security Council resolution.

"I called for co-ordination with the United States and South Korea to request the United Nations to hold an emergency Security Council meeting. Given that North Korea apparently has no intention of engaging in dialogue, Japan will coordinate with other countries to adopt a new Security Council resolution."

Image source, Reuters

Defence Expert Melissa Hanham has been looking at the picture released by North Korea, purportedly showing Kim Jong-un inspecting a missile-mountable H-bomb.

"There is no way of telling if this is the actual device that was exploded in the tunnel - it could even be a model - but the messaging is clear. They want to demonstrate that the know what makes a credible nuclear warhead," she writes for the BBC.

Follow Melissa on Twitter., external

Image source, afp

This is the picture that will be seen across the world today - Kim Jong-un standing very close to what seems to be a hydrogen bomb. Defence expert Melissa Hanham breaks down just what the picture tells us.

Image source, KCNA

People in China living near the North Korean border have been sharing videos of the tremors from the nuclear explosion shaking their homes.

The BBC's Yogita Limaye in Seoul says the latest test comes against a backdrop of heightened tensions in the region following multiple missile launches by North Korea, the last one conducted less than a week ago. That one flew directly over northern Japan, which called it an "unprecedented" threat.

"North Korea has been testing nuclear weapons since 2006 and even though repeated sanctions have been imposed on the country, Pyongyang has said it will not halt its nuclear programme," she adds.

You can read more about North Korea's extensive missile programme here.