SEOUL, South Korea — North Korea faced a barrage of global condemnation and calls for harsh sanctions today after it announced that it had set off an atomic weapon underground, a test that thrusts the secretive communist state into the elite club of nuclear-armed nations.

The United States, Japan, China and Britain led a chorus of criticism and urged action by the United Nations Security Council in response to the reported test, which fell one day after the anniversary of reclusive North Korean leader Kim Jong-Il's accession to power nine years ago.

South Korea's spy chief said there were possible indications the North was moving to conduct more tests.

The Security Council had warned North Korea just two days earlier not to go through with any test, and the Pyongyang government's defiance was likely to lead to calls for stronger sanctions against the impoverished and already isolated country.

White House spokesman Tony Snow said the U.S. government had not confirmed whether the North's claims of an underground nuclear test are true. But he said "a test would constitute a provocative act in defiance of the will of the international community and of our call to refrain from actions that would aggravate tensions in Northeast Asia."

A U.S. government official, who spoke on condition of anonymity because of the political sensitivity of the situation, said the seismic event could have been a nuclear explosion, but its small size was making it difficult for authorities to pin down.

There were conflicting reports about the size of the explosion.

South Korea's geological institute estimated the force of the explosion to be equivalent to 550 tons of TNT, far smaller than the two nuclear bombs the United States dropped on Japan during World War II. But Russian Defense Minister Sergei Ivanov said it was far more powerful, equivalent to 5,000 to 15,000 tons of TNT.

Only Russia said the blast was a nuclear explosion but the reaction of world governments reflected little doubt that they were treating the announcement as fact.

Although North Korea has long claimed it had the capability to produce a bomb, the test was the first manifest proof of its membership in a small club of nuclear-armed nations. A nuclear armed North Korea would dramatically alter the strategic balance of power in the Pacific region and would tend to undermine already fraying global anti-proliferation efforts.