The U.S. Geological Survey said it had detected a 4.2 magnitude quake in North Korea at 10:35 local timeon Monday, confirming a similar report from South Korea.
U.S. defense officials were still saying they could not confirm a nuclear test, however.
Pyongyang's official Korean Central News Agency (KCNA) said there was no leak or danger from its test, which diplomats had predicted would be conducted in the north of the country near the border with China.
``The nuclear test was conducted with indigenous wisdom and technology 100 percent,'' KCNA said.
``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.''
Analysts say North Korea probably has enough fissile material to make six to eight nuclear bombs but probably does not have the technology to devise one small enough to mount on a missile.
``If it is only one weapon, it would be a positive sign,'' former U.N. weapons inspector David Albright told CNN television.
``If they conducted two, three or four tests it would be more worrisome because it would suggest they were conducting ... a series of tests that would allow them to create a much better arsenal.''
RESPONSE FROM A CORNER
The U.N. Security Council last Friday urged North Korea not to carry out a test, warning Pyongyang of unspecified consequences if it did.
The nuclear test report pushed the dollar to a seven-month high against the yen and helped push oil above $60 a barrel. However, analysts did not expect long-term fallout unless the situation deteriorated further.
``The economy of North Korea is virtually closed from the rest of the world and its regional impact won't be very significant unless there was a major military confrontation,'' said Wang Qing, an economist at Bank of America in Hong Kong.
Abe, speaking in Seoul after arriving from an ice-breaking visit to Beijing, said Japan had no confirmation of a test by North Korea. However, the government's chief spokesman said in Tokyo that such a move would be a ``grave threat'' to Japan.
China, the closest North Korea has to an ally, described the nuclear test as ``brazen'' and called on its neighbor to stop any action that would worsen the situation.
Seoul and Beijing -- leery of instability on the Korea peninsula -- have previously cautioned against backing the North into a corner, but Tokyo backs a hard line toward Pyongyang.
However, all three agree that Pyongyang should end its nearly year-long boycott of six-country talks on ending its nuclear weapons program.
``I don't think North Korea is trying (for) an escalation that could lead to a military confrontation. ... I think they're trying to respond from a corner,'' Albright said.
North Korea announced its intention to test a nuclear device last week, saying its hand was forced by what it called U.S. threats of nuclear war and economic sanctions. But it said it would not be the first to use a nuclear weapon.
``North Korea is using this claim as a bargaining chip to gain leverage so that Washington will take them seriously,'' said Dewi Fortuna Anwar, a political analyst at the Indonesian Institute of Sciences and a former Indonesian presidential adviser.
A U.S. official, speaking on condition of anonymity, said North Korea gave China a 20-minute warning of its test and China immediately told the United States, Japan and South Korea.