North Korea conducted an underground nuclear test on Monday,
flying in the face of a warning from the UN Security Council and
opening its crippled economy to the risk of fresh sanctions.
South Korea's military ordered the army to step up a state of alert after Pyongyang announced its first-ever nuclear test, which brought unusual criticism from fellow communist China.
Pyongyang's move, which came about 30 minutes before Japanese Prime Minister Shinzo Abe landed in Seoul for a visit, could heighten regional tension and could deal a fresh foreign policy blow to US President George Bush ahead of mid-term polls.
The White House branded the act "provocative" and said it expected the UN Security Council to take immediate actions.
New Zealand Prime Minister Helen Clark says the test is a
provocative and destabilising act for the region.
North Korea's announcement pushed the dollar to an eight-month high against the yen and helped shove oil above $US60 a barrel. In Seoul, the won fell 1.5% to two-month lows and the main stock index tumbled as much as 3.6%.
The US Geological Survey said it had detected a 4.2 magnitude tremor in North Korea.
In a report from Moscow, Japanese broadcaster NHK quoted a Russian Defence Ministry official as saying it was "100% sure" that North Korea carried out a test at around that time.
Pyongyang's official Korean Central News Agency (KCNA) said there was no leak or danger from its test, which measurements by the Japan Meteorological Agency showed took place around Gilju, on the country's northeast coast and around 110 kilometres from the Chinese border.
"The nuclear test was conducted with indigenous wisdom and technology 100%," KCNA said.
"It marks a historic event as it greatly encouraged and pleased the KPA (Korean People's Army) and people that have wished to have powerful self-reliant defence capability."
Analysts say North Korea probably has enough fissile material to make six to eight nuclear bombs but probably lacks the technology to devise one small enough to mount on a missile.
Pyongyang did not indicate if there would be further tests. India and Pakistan, the last countries to conduct tests in 1998, carried out several each.
Gary Gibson of Australia's Seismology Research Centre estimated the blast at about one kiloton. That was dwarfed by India's biggest - around 45 kilotons - and the 10-kiloton bomb dropped on the Japanese city of Hiroshima in 1945.
The UN Security Council urged North Korea last week not to carry out a test, warning of unspecified consequences if it did.
Japanese Foreign Minister Taro Aso said Tokyo was considering further sanctions on North Korea and might push for a fresh Security Council resolution if the nuclear test were confirmed.
The Security Council was due to meet on Monday to officially nominate South Korean Foreign Minister Ban Ki-moon as next United Nations secretary-general, at which time it was likely to discuss North Korea's move.
A US official, speaking on condition of anonymity, said the North had given China a 20-minute warning of its test and Beijing had immediately alerted Washington, Tokyo and Seoul.
Although it is the closest reclusive North Korea has to an ally, China described the nuclear test as "brazen" and called on its neighbour to stop any action that would worsen the situation.
Seoul and Beijing - leery of instability on the Korea peninsula - have previously cautioned against backing the North into a corner, but Tokyo backs a hard line towards Pyongyang.
In a corner
However, all three agree Pyongyang should end its 11-month boycott of six-nation talks on ending its nuclear arms programme.
"I don't think North Korea is trying (for) an escalation that could lead to a military confrontation. ... I think they're trying to respond from a corner," former UN weapons inspector David Albright told CNN television.
North Korea announced its intention to test a nuclear device last week, saying its hand had been forced by what it called US threats of nuclear war and economic sanctions. But it said it would not be the first to use a nuclear weapon.
"North Korea is using this claim as a bargaining chip to gain leverage so that Washington will take them seriously," said Dewi Fortuna Anwar, a political analyst at the Indonesian Institute of Sciences and a former Indonesian presidential adviser.
Zhang Liangui, an expert on North Korea at the Central Party School, a top think-tank in Beijing, said it was unlikely Japan would seek to become a nuclear weapons state, because of US opposition, but other states may be encouraged to proliferate.
"It will be like America, where everybody thinks he has the right to own a gun," he said. "The first country to be encouraged by this will be Iran, and then other countries in the Middle East."