WASHINGTON (CNN)-- President Bush on Monday said North Korea's claim that it has tested a nuclear weapon is a threat to international peace and said the world "will respond."
"The transfer of nuclear weapons to states or non-state entities would be considered a grave threat to the United States," Bush said. "And we would hold North Korea fully accountable to the consequences of such action."
Bush said the U.S. was still trying to confirm whether North Korea had actually tested a nuclear weapon as it claimed earlier Monday.
He said he'd spoken with the leaders of South Korea, Russia, China and Japan, who agree on an "immediate response by the United Nations Security Council," set to meet Monday in New York.
China, a close ally of North Korea, denounced the claimed test as "brazen," and South Korea said it would respond "sternly" to a move that experts said raised fears of nuclear terrorism and a regional arms race.
North Korea's ambassador to the U.N. Pak Kil-yon said the council should "congratulate" North Korea's scientists and researchers "instead of [issuing] such notorious, useless and reckless resolutions or whatever the statement against" North Korea.
Pak said the test was "very, very successful" and will contribute "to the maintenance and guarantee of peace and security in the peninsula and the region."
When asked if North Korea plans to conduct further tests, Pak told reporters "that will be enough, you don't think so?"
Bush was expected to make a statement on the situation from the White House at 9:45 a.m. ET.
When North Korea warned last week that it intended to conduct a nuclear test, international analysts said it could unleash a regional arms race and give a virtual green light to Iran, which the United States suspects wants to develop nuclear weapons. Experts also fear North Korea may allow terrorists such as al Qaeda access to its nuclear weapons technology.
"This immediately affects the calculations of South Korea, Taiwan, Japan, all of whom might decide that they need to have their own independent nuclear arsenal as well," said international security analyst Joseph Cirincione of the Center for American Progress. "If North Korea gets away with this, Iran will be encouraged to go forward."
If confirmed, the test would be the first of its kind since Pakistan detonated an underground nuclear weapon in May of 1998, said Hans Kristensen of the Federation of American Scientists in Washington. North Korea would be the eighth nation to successfully conduct such a test, he said.
North Korea has recently test fired seven missiles, including a long-range ballistic missile in July, but it's unknown whether Pyongyang possesses the high-technology expertise to construct a nuclear weapon small enough for a missile delivery system.
On Wednesday, U.S. Assistant Secretary of State Christopher Hill said, "North Korea can have a future or it can have these weapons. It cannot have both."
Immediately following North Korea's announcement Monday, South Korean president Roh Moo-hyun said Pyongyang has "broke the trust of the international community.
Roh said it brought "a severe situation that threatens stability on the Korean Peninsula and in northeast Asia."
Abe told the same news conference his country would work "to make ways to implement action for a tough resolution."
Australian Prime Minister John Howard suggested financial, trade and travel sanctions, saying a "strong international response is called for."
CNN's Dan Rivers, speaking from the demilitarized zone between North and South Korea, said the key question now was what China -- which effectively allowed North Korea to exist economically -- would do.
The apparent nuclear test was conducted at 10:36 a.m. (1:36 a.m. GMT) in Hwaderi near Kilju city, South Korea's Yonhap news agency reported, citing defense officials.
South Korea's state geology research center detected a 3.58-magnitude "artificial earthquake" in a remote area of North Korea's North Hamgyeong Province, according to the news agency. Judging from the seismic tremor, the center said the power of the explosion was equivalent to around a half kiloton of TNT explosives, Yonhap reported.
Reports of the claimed test triggered global condemnation.
South Korea's Defense Ministry raised the military alert level.
"The field of scientific research in the DPRK (North Korea's official name) successfully conducted an underground nuclear test under secure conditions on October 9. ... at a stirring time when all the people of the country are making a great leap forward in the building of a great prosperous powerful socialist nation," said North Korea's state news agency, Korean Central News Agency.
CNN's Matthew Chance said that Moscow said Russian equipment in the area had confirmed an underground test.
Defense Minister Sergei Ivanov said that the force of the blast was five to 15 kilotons.
In Washington, White House Press Secretary Tony Snow said 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."
A U.S. military official told CNN that "something clearly has happened," but the Pentagon was working to fully confirm the report.
The U.S. Geological Survey Web site recorded a light 4.2-magnitude earthquake in North Korea at 10:35 a.m., about 385 kilometers (240 miles) northeast of the North Korean capital, Pyongyang.
"The nuclear test was conducted with indigenous wisdom and technology 100 percent. It marks a historic event as it greatly encouraged and pleased the KPA (Korean People's Army) and people that have wished to have powerful self-reliant defense capability," KCNA reported.
"It will contribute to defending the peace and stability on the Korean Peninsula and in the area around it."
China on Monday demanded Pyongyang stop any action that would worsen the situation, Reuters news service reports.
"The DPRK has ignored the widespread opposition of the international community and conducted a nuclear test brazenly on October 9," China's Foreign Ministry said in a statement on its Web site.
"The Chinese government is firmly opposed to this," the statement said.
In Tokyo, the prime minister's office said Japan had established a task force to address the situation. Chief government spokesman Yasuhisa Shiozaki said if a nuclear test was confirmed, Japan would "strongly protest" it.
Russian President Vladimir Putin condemned the claimed test, Russian news agencies reported, as Russia demanded North Korea rein in its nuclear program.
On Friday, the Security Council warned North Korea against performing a nuclear test, citing unspecified action if it should do so.
South Korean eyed for top U.N. role
The Security Council also has called on North Korea to return immediately to six-party talks with China, Japan, South Korea, Russia and the United States. The United States and its allies have been urging North Korea to rejoin the talks aimed at persuading the reclusive Communist nation to abandon its nuclear arms program.
On Monday, members of the U.N. Security Council were expected to select South Korean Foreign Minister Ban Ki-moon to succeed Kofi Annan as secretary-general of the world body.
In a straw poll last Monday, all but one of the 15 council members supported that choice, according to Chinese Ambassador Wang Guangya.
CNN's Jamie McIntyre, Barbara Starr, Sohn Jie-ae and Elise Labott contributed to this report
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