The hardline communist state, which stunned the world with its first atomic bomb test in October 2006, made good on its threat to stage another test after the Security Council censured it for an April rocket launch.

The North "successfully conducted one more underground nuclear test on May 25 as part of the measures to bolster up its nuclear deterrent for self-defence in every way," the official Korean Central News Agency (KCNA) said.

"The current nuclear test was safely conducted on a new higher level in terms of its explosive power and technology," it said.

The foreign ministry in China, the North's most important ally, said it was "resolutely opposed" to the test.

"China strongly demands that North Korea keeps its promise of denuclearisation and ceases all actions that could further worsen the situation," it said a statement.

Russia's foreign ministry said the test threatened regional stability, violated the Security Council's will and impeded non-proliferation efforts.

China and Russia -- both part of a six-nation forum working to persuade the North to give up its nuclear programmes -- had in the past resisted efforts at the UN to punish the North harshly over its nuclear activities.

The force of Monday's blast was between 10 and 20 kilotons, according to Russia's defence ministry, vastly more than the estimated one-kiloton blast three years ago.

Japan's Meteorological Agency said that based on recorded seismic activity, the energy level of the test was four times bigger than the last one.

Baek Seung-Joo of the Korea Institute for Defence Analyses told AFP that if rough estimates by some private analysts are right, "the power of the second blast is comparable to the bombs which hit Hiroshima and Nagasaki."

The Security Council, which sanctioned the North for its previous test, planned to meet Monday afternoon in New York.

The North also test-fired three short-range missiles Monday, South Korea's military said.

"North Korea's attempts to develop nuclear weapons, as well as its ballistic missile programme, constitute a threat to international peace and security," US President Barack Obama said in a statement.

"The danger posed by North Korea's threatening activities warrants action by the international community.

"By acting in blatant defiance of the United Nations Security Council, North Korea is directly and recklessly challenging the international community," Obama said.

The North informed the United States and China in advance of the test, a South Korean official said on condition of anonymity.

South Korea, in mourning over the weekend suicide of former president Roh Moo-Hyun, called its neighbour's atomic test an "intolerable provocation" and a serious threat to regional peace.

It put its military on heightened alert.

Japan's Foreign Minister Hirofumi Nakasone, after meeting South Korean counterpart Yu Myung-Hwan in Hanoi, called the test "a challenge to the whole of the international community."

"We, as the only atomic-bombed nation, need to take stern action," he said.

Japan asked UN Security Council chair Russia to call an emergency meeting.

The KCNA report did not say where the nuclear test was conducted. South Korean officials said a tremor was detected around the northeastern town of Kilju, near where the first test was staged.

Japan, South Korea and the United States -- along with China and Russia -- have been negotiating since 2003 to persuade the North to abandon its nuclear programmes in exchange for energy aid and security guarantees.

In a 2007 pact the North agreed to dismantle its nuclear plants. The deal bogged down last December over ways to verify its declared nuclear activities.

In April the North fired a long-range rocket for what it called a satellite launch. Many nations saw it as a disguised ballistic missile test, while the Security Council condemned the launch and tightened sanctions.

A defiant North vowed to conduct a second nuclear test as well as more ballistic missile launches unless the world body apologised.

It also announced that it was quitting the six-way talks, which are hosted by China, and would restart its plutonium-making programme.

Analysts believe the North has currently stockpiled enough plutonium for six to 12 small nuclear bombs. The first test was seen as only partially successful.

KCNA said Monday's test had resolved "scientific and technological problems arising in further increasing the power of nuclear weapons and steadily developing nuclear technology."

North Korea has frequently said it needs a nuclear deterrent to prevent any attack. It said Monday's test would "contribute to defending the sovereignty of the country and the nation and socialism and ensuring peace and security on the Korean peninsula and the region."

The North has expressed disappointment at the Obama administration, calling it no better than its predecessor.

"The second test was earlier than expected and reflects the North's growing anger at Washington," said Kim Yong-Hyun of Seoul's Dongguk University.

"Or some internal problems may be forcing Pyongyang to take a strong attitude."

North Korean leader Kim Jong-Il, 67, was widely reported to have suffered a stroke last August, prompting speculation overseas about the succession. The North's position has noticeably hardened since then.

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