President Bush on Monday condemned North Korea's reported nuclear test as a provocative act and called on the U.N. Security Council to take immediate action. Bush said the U.S. was working to confirm North Korea's claim.

"Nonetheless, such a claim itself constitutes a threat to international peace and security. The United States condemns this provocative act," the president said from the White House.

"Once again, North Korea has defied the will of the international community and the international community will respond," he said.

Raw Data: North Korean Statement on Alleged Nuke Test

Bush said he spoke with the leaders of China, Japan, South Korea and Russia since the test. Japan's Prime Minister Shinzo Abe and Bush agreed to take "decisive action" at the U.N. Security Council and called the reported test a grave threat to international security, Japan's Foreign Ministry said in a statement.

"All of us agreed that the proclaimed actions taken by North Korea are unacceptable and deserve an immediate response by the United Nations Security Council," Bush said.

Speaking at the United Nations, U.S. Ambassador to the U.N. John Bolton told reporters that the Security Council would be willing to work "24/7 if need be" to get a resolution out to respond to Pyongyang's actions.

Bolton said it is ironic that Security Council nominated South Korean Foreign Minister Ban Ki-Moon as the next U.N. Secretary-General just as North Korea tested a weapon.

"It's really quite an appropriate juxtaposition" to be nominating Ban while also meeting to consider action against Pyongyang.

"I can't think of a better way to show the difference in progress" between the two nations since they split 61 years ago, Bolton said.

Bush warned that the North Korean regime "remains one of the world's leading proliferators of missile technology," sending its materials to Iran and Syria.

"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 to the consequences of such action," the president said.

CountryWatch: North Korea | South Korea

A U.S. government official, who spoke on condition of anonymity because of 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.

North Korea is believed to have enough plutonium for as few as four and as many as about a dozen nuclear bombs. But until Sunday's apparent action, Pyongyang had never tested a device.

One intelligence official, speaking on condition of anonymity, said U.S. intelligence agencies detected an explosive event in North Korea with a force of less than a kiloton. Historically, the types of devices used in initial nuclear tests have yielded several kilotons of force. One kiloton is defined as the energy produced in an explosion of 1,000 tons of TNT.

The test doesn't come as a surprise to U.S. and international officials, who were warned last week that North Korea was planning on testing a nuclear weapon. U.S. intelligence has been closely watching several sites in North Korea that could be used for a nuclear test. Movements of people, automobiles, fencing and other items convinced some analysts last week that a test could come soon.

Just prior to the test, China was notified it was coming, and the Chinese then notified the U.S. embassy in Beijing, which passed the word to other U.S. officials, White House Press Secretary Tony Snow told reporters in a conference call Monday morning.

National Security Adviser Stephen Hadley notified Bush around 10 p.m. EDT Sunday, Snow said. South Korea's Yonhap news agency reported that the suspected test was conducted at 9:36 p.m. EDT Sunday.

Bush said he reaffirmed to U.S. allies South Korea and Japan that the U.S. "will meet the full range of our deterrent and security commitments."

In a written statement, House Speaker Dennis Hastert, R-Ill., denounced North Korea's action as "the desperate act of a criminal regime" and said the House would support Bush and the international community in condemning that country's "reckless decision."

Senate Majority Whip Mitch McConnell said the test, whether nuclear or not, is "a confrontational move" meant to destabilize the region.

"This test should demonstrate to other members of the United Nations Security Council, particularly those in North Korea's neighborhood, that a stronger stance is necessary to restore stability and protect the region and the world from this dangerous regime," McConnell, R-Ky., said.

Democrats too condemned the test, but also blamed Bush for failing to prevent it.

"Weapons of mass destruction pointed at our allies and strategic partners represents a shocking failure of President Bush's security policy, and a threat to the interests of peace and stability in the world," said Sen. John Kerry, D-Mass., a member of the Senate Foreign Relations Committee and 2004 Democratic presidential nominee.

"The recently passed defense authorization bill requires President Bush to appoint a high level coordinator for North Korean policy," said House Minority Leader Nancy Pelosi, D-Calif. "That appointment should be made immediately and other nations whose policies on North Korea have also so clearly failed, like China, must urgently develop new approaches as well."

The U.S. and its allies have been trying to lure North Korea back to stalled international efforts to persuade Pyongyang to scrap its nuclear weapons program.

Bush said North Korea's action on Sunday only raises tensions while "depriving the North Korean people of the increased prosperity and better relations with the world offered by the implementation of the joint statement of the six-party talks.

"Threats will not lead to a brighter future for the North Korean people, nor weaken the resolve of the United States and our allies to achieve the denuclearization of the Korean Peninsula," he said.

The Associated Press contributed to this report.