SEOUL, South Korea (AP) -- North Korea's claim to have tested a nuclear weapon drew widespread condemnation Monday, with the United States calling it potentially a "provocative act" and even close ally China strongly opposing it.

North Korea's official Korean Central News Agency said the communist country's first-ever nuclear test, an underground explosion, was successfully performed Monday "with indigenous wisdom and technology 100 percent."

The U.S. and Australia called for immediate U.N. Security Council action. British Prime Minister Tony Blair said the reported test was "completely irresponsible."

The White House, saying U.S. and South Korean intelligence had detected a seismic event at a suspected nuclear test site in North Korea, cautioned that a confirmed test would be a provocation detrimental to regional security.

"A North Korean nuclear 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," White House spokesman Tony Snow said in Washington.

"We expect the U.N. Security Council to take immediate actions to respond to this unprovoked act," Snow said. "The United States is closely monitoring the situation and reaffirms its commitment to protect and defend our allies in the region."

A Security Council resolution adopted in July after a series of North Korean missile launches imposed limited sanctions on North Korea and demanded the country rejoin international nuclear talks. The North immediately rejected the plea.

Beijing -- a longtime supporter of the North but also the host of international talks aimed at persuading the fellow communist country to give up its nuclear ambitions -- strongly criticized the move.

"China resolutely opposes the North Korean nuclear test and hopes that North Korea will return to the six-nation talks," according to a Chinese Foreign Ministry statement read out on state television news. "Upholding the stability of Northeast Asia is in the interests of all parties."

China, Japan, Russia, South Korea and the United States have held intermittent talks with North Korea since 2003 in hopes of getting Pyongyang to abandon nuclear weapons in exchange for aid and security guarantees.

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.

In London, Blair criticized the North for defying the international community.

"I condemn this completely irresponsible act by the government of the DPRK," Blair said in a statement issued by his office, referring to the North by the abbreviation of its official name, the Democratic People's Republic of Korea.

"The international community has repeatedly urged them to refrain from both missile testing and nuclear testing," he said. "This further act of defiance shows North Korea's disregard for the concerns of its neighbors and the wider international community."

Strong criticism also came from South Korea, which shares the world's most heavily armed border with the North.

"The North's nuclear test is a provocative act and the North must clearly assume all responsibility," said Kim Geun-tae, head of the ruling Uri Party, according to the party.

South Korea's presidential spokesman, meanwhile, said Seoul will "sternly respond."

Seoul's Defense Ministry said the military's alert level had been raised in response to reports of the test.

North and South Korea have faced off at the heavily armed demilitarized zone separating them since the 1950-53 Korean War ended in a truce, not a peace treaty.

Japan's new Prime Minister Shinzo Abe, who was in South Korea Monday for the first summit between the neighbors in a year, called for a coordinated and level-headed response.

"Although the test still needs to be confirmed, there must be a calm yet stern response," Abe said at a luncheon just prior to the summit. "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."

Abe, who assumed office just two weeks ago, was in Seoul after a summit in China, where he and President Hu Jintao agreed that a test by North Korea would be "intolerable" and vowed to work to persuade Pyongyang to return to multilateral talks aimed at getting it to abandon its nuclear ambitions.

Indonesia, Southeast Asia's largest country, condemned North Korea over its announced test, saying such a move would add to regional tensions and threaten stability.

"The Government of Indonesia reiterates its position that the recent nuclear test conducted by the Democratic People's Republic of Korea is unacceptable under any justifiable reason," a Foreign Ministry statement said.

Written by CNN.
- posted by Rosemary Welch@4:41 AM