SEOUL, South Korea – The United States and Australia demanded immediate U.N. Security Council action against North Korea for its reported nuclear test, while China condemned its ally for blatantly defying the world.
British Prime Minister Tony Blair called the move "completely irresponsible," and his government warned of serious consequences for the isolated regime.
• Raw Data: North Korean Statement on Alleged Nuke Test
The U.N. Security Council planned to discuss the crisis Monday, and the United States and Japan are likely to press for a resolution imposing additional sanctions on Pyongyang.
North Korea said it tested its first atomic bomb in an underground explosion Monday morning. The White House said U.S. and South Korean intelligence detected a seismic event at a suspected North Korean nuclear site and were trying to confirm Pyongyang's claims.
"A North Korean nuclear test would constitute a provocative act in defiance of the will of the international community," said White House spokesman Tony Snow. "We expect the U.N. Security Council to take immediate actions to respond to this unprovoked act."
"The United States is closely monitoring the situation and reaffirms its commitment to protect and defend our allies in the region," he added.
China, a longtime North Korea supporter and host of stalled international talks to persuade the fellow communist country to give up its nuclear ambitions, strongly condemned the act.
"China expresses its resolute opposition," the Chinese Foreign Ministry said. The North "defied the universal opposition of international society and flagrantly conducted the nuclear test."
Australian Prime Minister John Howard said his government would call on the U.N. Security Council to take "swift and effective action" against North Korea, including financial, trade and travel sanctions.
"But if the United Nations fails to act effectively against this outrage from North Korea, it will represent a further diminution of its authority," Howard said.
A Security Council resolution adopted in July after a series of North Korean missile launches imposed limited sanctions on North Korea and demanded that the reclusive communist nation suspend its ballistic missile program -- a demand the North immediately rejected.
The resolution bans all U.N. member states from selling material or technology for missiles or weapons of mass destruction to North Korea. It also prohibits all nations from receiving missiles, banned weapons or technology from the North, known as the Democratic People's Republic of Korea, or DPRK.
Britain warned there would be repercussions for the reported nuclear test.
"I condemn this completely irresponsible act by the government of the DPRK," Blair said in a statement issued by his office. "This further act of defiance shows North Korea's disregard for the concerns of its neighbors and the wider international community."
The European Union called for "a decisive international response to this provocative act." French Foreign Minister Philippe Douste-Blazy echoed the appeal.
"It is again up to the international community to react very firmly," Douste-Blazy told The Associated Press at the United Nations office in Geneva.
South Korea, which shares the world's most heavily armed border with the North, said it put its military on high alert.
North Korea has created "a severe situation that threatens stability on the Korean Peninsula and in northeast Asia," South Korean President Roh Moo-hyun told journalists after a summit with Japanese Prime Minister Shinzo Abe.
He said the test would make it difficult for Seoul to maintain its efforts to strengthen ties with its communist neighbor.
"This is a warning as well as my prediction," Roh said. "Under this situation, it's difficult for South Korea to maintain engagement policy."
Abe said the development and possession of nuclear weapons by North Korea would transform the security environment in the region.
"We will be entering a new, dangerous nuclear age," said Abe, who is facing his first major foreign policy test since his recent election.
Earlier Abe called for a coordinated and levelheaded response.
"It is important for Japan and South Korea, along with the United States and China, to work together and send a message to the world," he said.
Indonesia also condemned the reported test as "unacceptable under any justifiable reason."