08:38 GMT: We are now closing this Live Report on North Korea's nuclear test and the widespread condemnation that followed.
0830 GMT: China, North Korea's closest ally, has also criticised Pyongyang for deciding to conduct the test despite Beijing’s very clear opposition and warnings.
"We strongly urge the DPRK to honour its commitment to denuclearisation, and not to take any actions which might worsen the situation," the foreign ministry said on its website.
In the weeks before the test, Chinese state media had warned Pyongyang of a "heavy price" if it went ahead.
0820 GMT: The UN atomic agency has added its voice to the chorus of condemnation following Pyongyang’s nuclear test. North Korea has clearly violated UN resolutions, says Yukiya Amano, the head of the International Atomic Energy Agency.
0805 GMT: AFP’s South Korea Bureau Chief Giles Hewitt reports that president-elect Park Geun-hye has strongly condemned the test and says her administration will not tolerate a nuclear-armed North “under any circumstances”.
"North Korea's nuclear test is a grave threat to the Korean Peninsula and international peace, hampers inter-Korean trust-building and undermines efforts for peace," Park says.
Park, who takes office on February 25, had campaigned on increasing engagement with Pyongyang but now the test and the recent missile launch have put that promise to the test.
0757 GMT: Stephanie Kleine-Ahlbrandt, Northeast Asia director of the International Crisis Group think tank, says: "I think that China is very angry about this test," and that she expects "stronger reactions" from the Communist Party's new leader Xi Jinping than his predecessors.
But Wang Dong, a Northeast Asia expert at Peking University, has told AFP that Beijing will probably respond to the test with limited measures such as cutting off financial access, but it is likely to keep any unilateral measures under wraps to avoid antagonising Pyongyang.
In 2006 Beijing quietly reduced the oil supply upon which Pyongyang depends, two months after the regime fired a ballistic missile and one month before it tested its first nuclear bomb.
0745 GMT: AFP's correspondent in Seoul, Nam You-Sun, reports that an anti-North Korean protest has been held in downtown Seoul following the nuclear test.
“Furious protestors covered a North Korean national flag with black and red paint and wrote ‘Strike Immediately’ on two other flags, one of them covered with pictures of Ri Sol-Ju and Kim Jong-Un, as well as a nuclear symbol.
“The South Koreans were shouting "Revenge!”
Nam says the South Korean police have dispatched SWAT patrol teams around the US embassy in downtown Seoul, along with explosive-detecting sniffer dogs. The reason for the move is still unclear.
0732 GMT: More of that reaction from London. "The UK will begin urgent consultations with (UN) Security Council partners calling for a robust response to this latest development," the Foreign Office says.
0725 GMT: The English-language statement by North Korea's official Korean Central News Agency, announcing the nuclear test, says: "The test was carried out as part of practical measures of counteraction to defend the country's security and sovereignty in the face of the ferocious hostile act of the U.S. which wantonly violated the DPRK's legitimate right to launch satellite for peaceful purposes."
It also says that North Korea's nuclear deterrence "has become diversified". Not immediately clear what that means.
0719 GMT: It may be early in the morning in London, but Britain is the next to wade in, calling for a "robust response".
0712 GMT: Among the first nations to condemn the nuclear test was old ally Russia, with Moscow saying it considers the test a violation of UN Security Council resolutions.
Russia's monitoring of radiation levels in the Far East of Russia neighbouring North Korea show normal conditions, Interfax reports.
0700 GMT: US President Barack Obama says North Korea's "provocative" test will not make it more secure and has vowed strong action in response.
The nuclear test has thrust Obama into a new overseas crisis just as he was about to use his annual State of the Union address to focus on jobs.
0655 GMT: AFP photographer Adek Berry in Jakarta, Indonesia, reports on how the North Korean nuclear test was recorded at the Indonesian Meteorology, Geophysics and Climatology Agency.
"Big screens zoomed in on the site in North Korea, registering seismic activity of 5.0. Everyone knew what it was, what had happened, but no-one wanted to say it. The staff did not want to talk about politics. But, it's not a standard earthquake site, so everyone knew."
0645 GMT: UN leader Ban Ki-moon has condemned the nuclear test as a "deeply destabilizing" provocation.
The UN, which called closed talks for 9:00 am New York time (1400 GMT), passed a resolution last month threatening "significant action" against Pyongyang in the event of a new nuclear test or missile launch.
0640 GMT: Japanese military aircraft are to take samples of air to detect radiation in the wake of North Korea's nuclear test, officials have told the media.
The Air Force is to dispatch aircraft from three bases in northern, central and southwestern Japan, later today.
0635 GMT: Japanese Prime Minister Shinzo Abe has condemned North Korea's nuclear test, saying it contravened international declarations.
0630 GMT: AFP photographer Mark Ralston, covering the story from Dandong, northern China, says the number of Chinese paramilitary soldiers guarding the bridges to North Korea and the shared border river has doubled.
“Security is tighter on the China side of the river. On the North Korean side the streets appear to be very quiet, everyone has disappeared. But that could be due to the terrible weather, there is heavy snow here today.”
0610 GMT: Spy agencies are evaluating a "seismic event" in the Stalinist state, the office of the Director of National Intelligence has told the media. It was the first US reaction so far to North Korea's nuclear test.
0555 GMT: A report by the official North Korean Central News agency says a third nuclear test was successfully staged "as part of measures to protect our national security and sovereignty against the reckless hostility of the United States that violated our republic's right for a peaceful satellite launch."
The TV report went on to say: "It has been confirmed that the high-level nuclear test, unlike in the past, had more explosive power and involved a more miniaturised and lighter atomic bomb and was staged safely and perfectly and had no negative impacts to the surrounding environment.
"The latest nuclear test will powerfully bolster the struggle of our military and people in the mission to build a powerful and prosperous country..."
0545 GMT: North Korea has confirmed it conducted a 'successful' nuclear test.
0540 GMT: A diplomat at the United Nations in New York has told AFP that "the Chinese gave the North Koreans a strong warning against carrying out a test as it became apparent that it was imminent."
"What the North Koreans have done now is a big challenge to the Chinese. There have been consultations in recent days, and in all likelihood China, Russia and the United States will quickly agree that tough action now has to be taken," the envoy added speaking on condition of anonymity.
0532 GMT: Experts have told AFP that despite North Korea’s nuclear tests over the years, Pyongyang still has some ways to go to shrink a warhead to the size needed to fit on a rocket and to develop a genuine intercontinental ballistic missile that could threaten the US mainland.
The first priority for the international community today will be to determine the precise nature and yield of the blast and what it reveals about the technical level of the North's nuclear weapons programme.
0522 GMT: AFP's Seoul bureau says that if a nuclear test is confirmed, the even more problematic question arises of how the international community should respond to a country that appears immune to coercion. North Korea has survived years of tough sanctions.
South Korea is the current president of the 15-country UN Security Council and has been calling for strong action against its arch-rival neighbour in the event of a nuclear test.
0508 GMT: The international nuclear test monitoring agency CTBTO, based in Vienna, says the blast recorded in North Korea is similar to earlier nuclear tests.
It also warns that "if confirmed as a nuclear test, this act would constitute a clear threat to international peace and security."
0502 GMT: AFP’s photographer in Beijing, Ed Jones, standing outside the North Korean embassy, says all is quiet.
“There is one guard on duty, and no vehicle/staff movements for the moment. A few other media started to arrive after shortly after I got here”.
Images showing Kim Jong-un and rocket launches in North Korea are displayed on a board outside the embassy for passers-by and the slowly gathering world media to admire.
0450 GMT: The 15-nation UN Security Council is expected to meet today in New York to discuss the suspected test.
A UN Security Council diplomat has told AFP that North Korea ignored a "strong warning" from China against staging another nuclear test.
As the North's only major ally and economic lifeline, China has long shielded Pyongyang from harsh global sanctions. But recent editorials in Beijing's state-run newspapers have shown increasing impatience with North Korea.
0445 GMT: South Korea has revised its earlier estimate of the explosive yield from the suspected blast in North Korea to between six and seven kilotons, rather than 10 kilotons or more.
When North Korea conducted a nuclear test in 2006, the explosive yield was recorded at one kilotons, and in 2009 between two and six kilotons.
Just for comparison the blast yields from the bombs dropped on Nagasaki and Hiroshima were 13 kilotons and 22 kilotons.
0436 GMT: South Korean officials believe the recorded quake was caused by a nuclear test in North Korea, AFP's bureau in Seoul reports.
The defense ministry is also "keeping an eye on the possibility there might be an additional nuclear test," correspondent Nam You-Sun says.
0430 GMT: Seismic data shows the suspected explosion in North Korea had a yield of at least 10 kilotons, signalling "enormous destructive power", according to South Korea's defence ministry.
04:21 GMT: It appears China and the United States were given advance warning by North Korea that it would conduct its third nuclear test today, according to the South Korean defence ministry.
0413 GMT: Experts have told AFP that even with sophisticated seismic monitoring and "sniffer" planes capable of detecting radioactive fallout, external analysis will provide only limited information on the test, especially if it was well-contained.
0408 GMT: The reported quake in North Korea has come after Pyongyang promised a "higher-level" nuclear test, fuelling speculation it would be of a uranium device, compared to the plutonium ones detonated in 2006 and 2009.
A uranium test would confirm suspicions that North Korea has been secretly enriching weapons-grade uranium for years and if a nuclear test is successful it would allow Pyongyang to significantly expand its small nuclear arsenal.
0404 GMT: Japan's government has called an emergency top level security meeting to discuss the suspected detonation of a nuclear device in North Korea. If it is confirmed as a nuclear test, it would throw down a stark security and diplomatic challenge to US President Barack Obama at the start of his second term, and to regional neighbours China, Japan and South Korea, all of which have new or incoming leaders.
0357 GMT: China's Earthquake Network Center says the quake recorded in North Korea was a "suspected explosion", and occurred at a "focal depth of 0 km".
0351 GMT: Top officials in Japan say they believe Pyongyang has detonated a nuclear device in its far northeast.
"We believe that there is a possibility that North Korea carried out a nuclear test, looking at past cases," said Chief Cabinet Secretary Yoshihide Suga.
0346 GMT: Scientists in Japan say the earth tremor measured in the northeast of the Korean peninsula was "different from a normal earthquake".
A posting on Prime Minister Shinzo Abe's Facebook page says the disturbance was shallow and occurred in almost exactly the spot where a quake was recorded at the time of the last nuclear test in May 2009.
0342 GMT: In response to the suspected test, AFP's correspondent in Seoul Nam You-Sun reports that South Korean president Lee Myung-Bak has called a National Security Council meeting at 0400 GMT.
The South Korean defense ministry has also raised the country's alert level and together with the United States has raised the Watch Condition level against the north.
0337 GMT: An "artificial earthquake of 5.1 magnitude" has been detected in Kilju county, where the Punggye-ri test site is located, according to Yonhap news agency.
The US Geological Survey measured it as a 4.9-magnitude quake at a very shallow depth of just one kilometre (0.6 miles).
Reports say that South Korea has detected an "artificial earthquake" at North Korea's nuclear test site, suggesting Pyongyang had gone ahead with a threatened atomic test.
WELCOME to AFP's Live Report on the suspected nuclear test in North Korea.
Just hours before the suspected test, North Korea's ruling party called for the staging of a "high-intensity" action and further long-range rocket launches.
Recent satellite imagery analysis of the North's remote nuclear test site, as well as US and South Korean intelligence reports, all concluded that the North had completed all necessary test preparations.
Pyongyang has said any test would be a direct response to the UN Security Council's adoption of a US-proposed resolution that condemned the North's rocket launch in December and expanded existing sanctions.
North Korea insists the rocket launch was a scientific mission aimed at putting a satellite into an orbit.
But the United States and many other countries viewed it as a disguised ballistic missile test, banned under UN resolutions triggered by Pyongyang's previous nuclear tests in 2006 and 2009.