SEOUL, South Korea (CNN) -- North Korea claimed it conducted a successful underground nuclear test Monday, according to the country's official Korean Central News Agency (KCNA).
China, a close ally of North Korea, denounced the claimed test as "brazen" and South Korea said it would respond "sternly." The United States said a test would constitute a "provocative act."
South Korea's president said Pyongyang's claimed test "broke the trust of the international community."
President Roh Moo-hyun said it brought "a severe situation that threatens stability on the Korean Peninsula and in northeast Asia."
South Korea would "react sternly and calmly" with "appropriate measures" in close cooperation with the international community, he told journalists after a summit with new Japanese Prime Minister Shinzo Abe.
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.
Reports of the claimed test triggered global condemnation (Full story).
Senior U.S. officials said the United States is consulting with allies around the world and would push for sanctions Monday at a 9:30 a.m. (1:30 p.m. GMT) meeting of the U.N. Security Council in New York.
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," KCNA reported.
CNN's Matthew Chance said that Moscow said Russian equipment in the area had confirmed an underground test.
In Washington, White House Press Secretary Tony Snow responded to the reports in a conference call with reporters.
"U.S. and South Korean intelligence detected a seismic event Sunday at a suspected nuclear test site. North Korea has claimed it conducted an underground nuclear test," 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," Snow added.
A senior U.S. official said China was given a 20-minute warning ahead of the test and in turn passed the information along to the United States, Japan and South Korea.
A U.S. military official told CNN that "something clearly has happened," but the Pentagon was working to fully confirm the report.
Other senior U.S. officials said they also believed the test took place, citing seismic data that appears to show one.
The U.N. Security Council was already scheduled to vote on the nomination of South Korean Foreign Minister Ban Ki-moon to be the successor to U.N. Secretary-General Kofi Annan. The body is still expected to vote on Ban before moving onto North Korea.
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.
South Korean intelligence officials said a seismic wave of magnitude-3.58 had been detected in North Hamkyung province, according to Yonhap.
"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's reaction
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.
High-level South Korean officials, meanwhile, were meeting Monday after intelligence of the suspected test was received.
"President Roh Moo-hyun called in an emergency meeting of related ministers on Monday to discuss the North Korean nuclear issue," said Yonhap, quoting Foreign Ministry spokesman Choo Kyu-ho.
"The meeting comes as there has been a grave change in the situation involving the North's nuclear activity."
According to KCNA, there was no radioactive leakage from the site.
The U.N. Security Council is expected to discuss the North Korean issue on Monday, and the United States and Japan are likely to press for a resolution imposing additional sanctions on Pyongyang, The Associated Press reported.
On Friday, the Security Council warned North Korea against performing a nuclear test, citing unspecified action if it should do so.
It also called on North Korea to return immediately to the six-party talks with China, Japan, South Korea, Russia and the United States.
'Serious provocation'
The report of a North Korean nuclear test came as Japan's new Prime Minister Shinzo Abe arrived in Seoul for meetings with President Roh Moo-hyun to address the nuclear issue as well as address strains in relations between the two countries over territorial and historical disputes.
Also Monday, North Korea accused South Korea of committing a serious provocation by firing warning shots during a weekend incident in which the South says soldiers from the communist North crossed over their border.
On Monday, members of the U.N. Security Council are 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.
John Bolton, U.S. ambassador to the United Nations, refused to discuss the outcome of the vote, but said: "I think it was sufficiently clear that all members of the council agreed to move to a formal vote on Monday night," he said. The announcement would be made Tuesday, he said.
