UNITED NATIONS (Reuters) - The U.N. Security Council unanimously condemned North Korea’s latest nuclear test on Monday, saying it was a “clear violation” of a previous resolution passed in 2006 after Pyongyang’s first atomic test.

After an emergency meeting lasting under one hour, the council issued a nonbinding statement expressing strong opposition and said it would “start work immediately on a Security Council resolution on this matter.”

U.S. Ambassador to the United Nations Susan Rice called the nuclear test “a grave violation of international law” and said Washington would seek a “strong resolution with strong measures.” She declined to predict whether the 15-member council would impose further sanctions on Pyongyang, but said work was to begin on the resolution on Tuesday.

Britain, France, and Japan are expected to push for new sanctions. But Russia and China are seen as more reluctant although they did agree to punitive actions after the 2006 test in resolution 1718.

Earlier on Monday, China, the North’s neighbor and long-time benefactor, said it was “resolutely opposed” to the test.

Russia called the North Korean nuclear test a threat to regional stability, and at the United Nations Russian Ambassador Vitaly Churkin told reporters it was “very serious and needs to have a strong response.”

U.S. President Barack Obama strongly condemned Pyongyang’s action and called for a strong international response.

“North Korea’s nuclear and ballistic missile programs pose a grave threat to the peace and security of the world, and I strongly condemn their reckless action,” Obama said at the White House. “The United States and the international community must take action in response.”

European and Asian stock markets generally shrugged off news of the nuclear test and missile launches, as investors focused on company news in thin trading. The yen took a hit on the news. U.S. and British markets were closed for a holiday.

SIZE OF DETONATION

The size of North Korea’s second nuclear test was not yet confirmed.

Russia said the blast was about equal in power to the U.S. atomic bomb dropped on the Japanese city of Nagasaki in World War Two, or about 20 times larger than the North’s one kiloton test in 2006.

The United States and the Vienna-based Comprehensive Nuclear Test Ban Treaty Organization saw it as smaller. Although analysis is continuing, a senior Obama administration official said it suggests an explosive yield of “approximately a few kilotons TNT.” The CTBTO said the magnitude was measured at measuring 4.52 on the Richter scale, while in 2006 it was 4.1.”

Officials in Washington and Beijing said North Korea had warned their governments of the test about an hour before the Monday detonation (9 p.m. EDT Sunday/0100 GMT Monday) but Japan said it was not given advance notice.

The Obama administration official said President Barack Obama was notified of the blast at 11:15 p.m. EDT (0315 GMT) after the U.S. Geological Survey noticed seismic activity.

Raising tensions further, North Korea test-fired three short-range missiles hours later, South Korea’s Yonhap news agency said.

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PROLIFERATION CONCERNS

U.S. Secretary of State Hillary Clinton spoke to Japanese Foreign Minister Hirofumi Nakasone and South Korean Foreign Minister Yu Myung-hwan, and planned to speak later with her Chinese and Russian counterparts, State Department spokesman Ian Kelly said.

The test confounded the international community, which has for years tried a mixture of huge aid pledges and tough economic sanctions to persuade the impoverished North to give up efforts to build a nuclear arsenal.

It was also bound to raise concerns about proliferation, a major worry of the United States which has in the past accused Pyongyang of trying to sell its nuclear know-how to states such as Syria.

Iran, which the West accuses of secretly developing atomic weapons and which conducted a missile test last week, said it had no missile or nuclear cooperation with North Korea.

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Analysts said North Korea’s test will force Washington to acknowledge that its leverage over the unpredictable state is at best limited. The United States must hope China will put pressure on Pyongyang, despite China’s fear of destabilizing its poor, secretive neighbor.

MARKETS BRIEFLY UNSETTLED

South Korea’s main stock market fell more than 6 percent at one stage but its decline was short-lived. Analysts said investors were used to the North’s saber-rattling and would likely panic only if there was military conflict on a peninsula where 2 million troops face each other.

North Korea already is so isolated there is little left with which to punish an autocratic government that has been ready to take dealings with the outside world to the brink.

Its leaders repeatedly stress the threat from the United States to justify heavy spending on the military that keeps them in power but which has meant deepening poverty, and at times famine, for many of its 23 million people.

The official KCNA news agency said North Korea had “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 test comes as speculation has mounted that leader Kim Jong-il, his health uncertain after reports of a stroke last year, wants to further strengthen his iron grip on power so he can better secure the succession for one of his three sons.

North Korea had for weeks threatened to conduct the test in response to tighter international sanctions following its April launch of a rocket, widely seen as a disguised long-range missile that violated U.N. resolutions.