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-defense 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 denuclearization 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 programs - had in the past resisted efforts at the U.N. to punish the North harshly over its nuclear activities.

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

Baek Seung-Joo of the Korea Institute for defense Analyses said 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 yesterday afternoon in New York. The North also test-fired three short-range missiles yesterday, South Korea's military said.

"North Korea's attempts to develop nuclear weapons, as well as its ballistic missile program, constitute a threat to international peace and security," U.S. 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 neighbor'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."

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 yesterday'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 yesterday'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."