Beijing ? North Korea’s official news agency said today the nation performed a successful nuclear test, defying warnings from the U.N. Security Council and a threat from the United States of military action.

The test was carried out without any radioactive leakage, the Korean Central News Agency reported.

South Korea’s semiofficial Yonhap news agency cited intelligence officials saying they had detected a 3.58-magnitude seismic tremor in North Korea. The Bush administration offered no immediate confirmation of the blast. CNN said the U.S. Geological Survey reported no seismic tremors from a blast.

Yonhap said the test appeared to occur in North Hamgyeong province around 10:36 a.m. (8:36 p.m. CDT Sunday).

“The nuclear test is a historic event that brought happiness to our military and people,” the North Korean news report said.

The sketchy report sent political tremors across East Asia and beyond.

South Korean President Roh Moo-hyun convened his security team in an urgent session, just as Japanese Prime Minister Shinzo Abe landed in Seoul for a one-day state visit aimed in part on resolving the Korean Peninsula nuclear crisis.

Abe said hours earlier in Beijing that Japan and China would “never tolerate” a nuclear test by North Korea, although Chinese officials did not use such firm language.

Ruled by Kim Jong Il, one of the world’s last totalitarian leaders, North Korea said only last week that it would test a nuclear weapon. Pyongyang announced in February 2005 that it had succeeded in making a nuclear bomb. Foreign intelligence officials say North Korea may have anywhere from six to a dozen nuclear bombs now.

South Korean tourists look at a defaced cartoon of North Korean leader Kim Jong Il put up Sunday during an anti-North Korea rally at the Imjingak Pavilion, near the demilitarized zone of Panmunjom, South Korea. North Korea today said it had conducted an underground nuclear test, despite strong opposition from its neighbors, including China, its closest ally.

A nuclear test, if confirmed, would severely jolt East Asia and generate pressure within the United States and from Japan to take military action to curb North Korea’s military program.

The U.N. Security Council is expected to discuss the reported North Korean test today, and the United States and Japan are likely to press for a resolution imposing additional sanctions on Pyongyang.

North Korea has said it is building nuclear weapons to forestall a military attack by the United States to topple the Kim regime, which President Bush once described as part of an axis of evil.

News of the alleged test gave added urgency to Abe’s visit to Seoul. Earlier today, Abe concluded a one-day summit with China’s three top leaders that put a thaw in chilly relations between regional rivals China and Japan. It was the first summit of its kind since 2001.

Abe, who has been in office for less than two weeks and is a staunch conservative, was given red carpet treatment in Beijing, in an apparent snub to North Korea. China and Japan capped the summit with a statement that they had reached “a turning point” in their tense relations.

Abe said late Sunday that China’s leaders agreed that any attempt by North Korea to test a nuclear weapon, as it vowed to do Tuesday, would be unacceptable.

“In my meeting with President Hu Jintao, we saw eye to eye that North Korea’s announcement of its intent to conduct a nuclear test can never be tolerated because it is a great threat to the peace and security of East Asia,” Abe said.

The summit marked a dramatic effort by China and Japan to put a thaw in their private cold war.

Sino-Japanese relations sunk to their nadir in April 2005 when protesters in China trashed Tokyo’s diplomatic facilities and Japanese stores in weeks of demonstrations. The tensions came despite growing economic ties between the two nations.

China is now Japan’s major trade partner, surpassing the United States. Abe noted that Japanese investment has created 10 million jobs in China.

China came to North Korea’s aid during the 1950-1952 war on the Korean Peninsula, and relations between the two neighbors were once described as like “lips and teeth.” But China is upset that North Korea may be sparking a regional nuclear arms race.

China supported a U.N. Security Council statement Friday urging Kim’s regime to abandon plans for the nuclear test and return to six-nation talks aimed at dismantling its nuclear arms program.