The U.N. Security Council added North Korea to the agenda of a meeting scheduled Monday.
President Bush condemned the test, calling it a "provocative act," and warned the Asian country against transferring the technology.
"The transfer of nuclear weapons or material by North Korea to states or non-state entities would be considered a grave threat to the United States and we would hold North Korea fully accountable for consequences of such action," he said.
The announcement was made in Pyongyang's official Korean Central News Agency, which claimed the test was a "historic event." If confirmed, North Korea would become the eighth country to successfully test a nuclear bomb.
"The nuclear test was conducted with indigenous wisdom and technology 100 percent," the news agency said, according to Reuters.
Although American officials said they could not confirm that a test occurred, the U.S. Geological Survey said it had detected a tremor of 4.2 magnitude on the Korean Peninsula.
South Korea, which has lived with an uneasy truce and failed efforts at reconciliation for more than half a century, said they detected an explosion about 10:35 a.m. local time.
They said the source was in North Hamgyong Province, an area where American satellites have been focused for several years on a variety of suspected underground test sites.
North Koreas allies reacted immediately. China, the communist country's closet supporter, called it a "flagrant and brazen" violation of international opinion and said it "firmly opposes" North Korea's conduct.
Across the North Korean border in Russia, President Vladimir Putin said, "Russia absolutely condemns North Korea's nuclear test."
Russian Defense Minister Sergei Ivanov said that the Russian military confirmed the test.
The United States is pushing Russia and China to cut off trade and oil, which have been propping up the erratic and fragile rule of President Kim Jong-Il.
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