North Korea has announced it has carried out its first test of an atomic bomb, defying international efforts to keep the secretive regime from joining the world's nuclear powers.

News of the underground blast sent shudders through capitals across the globe and sparked immediate condemnations, crisis talks and calls for tough sanctions from the UN Security Council, expected to meet later in the day.

The blast also underlined North Korean leader Kim Jong-Il's readiness to test the mettle of the international community, which warned just last week that Pyongyang would pay a heavy price if it tested a nuclear weapon.

One of the most isolated and impoverished nations in the world, North Korea called the blast a "historic event" that had been carried out safely for the betterment of security and peace.

But nations around the globe, from arch-foe the United States to North Korea's main ally China, reacted with worry and dismay. Stock markets in Asia plummeted amid fears the test would spark a new nuclear arms race.

"We expect the Security Council to take immediate action to respond to this unprovoked act," said White House spokesman Tony Snow. China said the test was "brazenly" in defiance of international demands.

Making good on last week's vow to carry out the test because of what it called the threat of war from the United States, North Korea said the test had been successfully carried out under secure conditions.

"It marks a historic event as it greatly encouraged and pleased the (North Korean army) and people that have wished to have powerful self-reliant defence capability," the official KCNA news agency said.

"It will contribute to defending the peace and stability on the Korean peninsula and in the area around it," KCNA said.

Little is publicly known about the nuclear programme of the North, which first announced last year that it had developed a nuclear weapon. The CIA has said North Korea probably had several nuclear warheads.

Officials in neighbouring South Korea quoted by Yonhap news agency said the blast appeared to have been carried out at 0136 GMT at Hwadaeri near North Korea's northeast coast.

The South Korean presidential office said the state intelligence agency had detected a 3.58-magnitude seismic tremor at the time.

A South Korean lawmaker, citing intelligence officials, said it appeared to have been conducted in a horizontal tunnel in a 360-metre (1,200-foot) mountain near a missile base in the Stalinist country's northeast.

The test came just while new Japanese Prime Minister Shinzo Abe, a hawkish politician who rose to prominence at home by taking a hard line on the North Korean regime, was to hold his first summit with his South Korean counterpart.

Abe and South Korean President Roh Moo-Hyun were meeting in Seoul for talks that had been expected to focus on finding a way to dissuade North Korea from going ahead with the test.

"North Korea's nuclear weapons test can never be pardonable. But we should collect and analyze more intelligence on the matter in a cool-headed manner," Abe said.

Roh called an emergency meeting of related ministers to discuss counter-measures, while Japan and South Korea launched crisis talks on the issue.

"Our government will sternly deal with this in accordance with the principle that it will not tolerate North Korea's possession of nuclear weapons," South Korean presidential spokesman Yoon Tae-Young said.

After years of dispute over the North, progress appeared in September last year, when Pyongyang said at six-nation talks it would abandon its programme in exchange for security and other guarantees.

But within two months it said it was boycotting further talks after the United States imposed unilateral financial sanctions on the North over alleged money laundering and counterfeiting of US currency.

Pyongyang was then slapped with limited sanctions by the Security Council after test-firing seven missiles in July this year -- one of them believed to be eventually able to reach US soil.

On Monday, the United States quickly warned that it would come to the defence of Japan, which has been officially pacifist since World War II and falls under the US strategic defence umbrella.

"The United States is closely monitoring the situation and reaffirms its commitment to protect and defend our allies in the region," said Snow, the White House spokesman.

Bush has made the fight against nuclear proliferation a centrepiece of his foreign policy, especially since the September 11, 2001 attacks on the United States.

Christopher Hill, the lead US envoy on North Korea, said last week: "We are not going to live with a nuclear North Korea. We are not going to accept it."

But it was not immediately clear what options the United States had to deal with Pyongyang.

North Korea has repeatedly argued it needs nuclear weapons to deter any attack from the United States, which it fears will try to topple one of the last pure Communist regimes in the world.

In announcing plans for the test last week, North Korea said it was "at the crossroads of life and death."

There are about 29,000 US troops stationed in South Korea, alongside 650,000 South Korean troops that face off against 1.2 million North Korean troops across one of the most heavily militarised borders in the world.

PHOTO CAPTION

North Korean leader Kim Jong-il is seen in this undated photo released by Korea News Service in Tokyo August 14, 2006. (Reuters)