SEOUL -- North Korea on Monday appeared to have detonated a nuclear device, its second such test in three years.
The U.S. Geological Survey said it detected a magnitude-4.7 earthquake in the region, and South Korea's Cabinet convened in an emergency session, according to Yonhap, South Korea's semi-official newswire.
Yonhap also said the North had confirmed the test.
Trading on South Korea's stock market was briefly suspended when the benchmark index plunged after the news was reported.
Experts here have long been predicting a second nuclear test.
North Korea first detonated a nuclear device in October 2006, but the test is thought to have produced a lower explosive yield than had been hoped for.
As North-South relations struggle, the North is busily engaged in accelerating tensions in the region - with harsh rhetoric, the detention of U.S. and South Korean citizens, and a recent ballistic-missile test.
Although North Korea surprised experts by not warning that it was about to test, analysts said they doubted the detonation was aimed to coincide with the aftermath of the apparent suicide of former South Korean President Roh Moo-hyun.
Mr. Roh had been the most conciliatory of South Korean presidents, and relations with the North went downhill after when his five-year term ended and voters elected hard-liner Lee Myung-bak as his successor.
"I think the timing is just coincidence; the North Koreans have a number of reasons to do this," said Dan Pinkston, who heads the International Crisis Group's Seoul office.
"North Korea is desperate to talk with the U.S. before it applies sanctions, which could have a major effect on its international banking operations," added Choi Jin-wook of the Korea Institute of National Unification.
North Korea has long sought bilateral talks with the United States. But its behavior in the past year and a half - after changes of government in both Seoul and Washington and the illness of its leader, Kim Jong-il - has been increasingly threatening.
North Korea threatened last month to conduct a nuclear test unless the United Nations apologized for its condemnation of an April 5 launch of a multistage rocket.
The U.N. Security Council denounced the rocket launch and threatened to impose tighter sanctions on the isolated communist state.
The North said the rocket was part of a civilian space program, but analysts said the technology is virtually identical to that needed to build a nuclear-tipped missile.
The North conducted its first nuclear test in 2006 and is thought to have enough plutonium to make a half-dozen atomic bombs.
After the latest missile test, the North pulled out of six-nation talks over its nuclear future and announced that it would begin reprocessing fuel from its nuclear reactor to make more plutonium.
It then threatened to conduct its second nuclear test and to conduct further tests to develop an intercontinental ballistic missile.
Under a 2007 six-nation deal, North Korea agreed to disable its main nuclear complex in Yongbyon, north of Pyongyang, in return for 1 million tons of fuel oil and other concessions. In June 2008, North Korea blew up the cooling tower there in a dramatic show of its commitment to denuclearization.
But disablement came to halt a month later as Pyongyang wrangled with Washington over how to verify its past atomic activities. The latest round of talks, in December, failed to push the process forward.
The negotiations involve China, Japan, the two Koreas, Russia and the United States.