The country's official Korean Central News Agency said the test was performed and that there had been no radioactive leakage from the site.
"The nuclear test is a historic event that brought happiness to the our military and people," KCNA said.
"The nuclear test will contribute to maintaining peace and stability in the Korean peninsula and surrounding region."
South Korean President Roh Moo-hyun has convened a meeting of security advisers over the issue, South Korea's Yonhap news agency reported, and intelligence over the test has been exchanged between concerned countries.
The news agency reported that North Korea nuclear test was conducted at 10:36 A M (0706 IST) in Hwaderi near Kilju city, citing defense officials.
The North said last week it would conduct a test, sparking regional concern and frantic diplomatic efforts aimed at dissuading Pyongyang from such a move. North Korea has long claimed to have nuclear weapons, but had never before performed a known test to prove its arsenal.
North Korea's nuclear test deplorable: Pranab
Deploring North Korea's nuclear test, Defence Minister Pranab Mukherjee today said India was strongly opposed to any clandestine transfer of nuclear technology - a veiled reference to the reported secret nuclear co-operation between Islamabad and Pyongyang.
India deplores N Korean nuclear tests
"We are deeply concerned at the reported nuclear tests conducted by DPR Korea," external affairs ministry spokesman Navtej Sarna said.
US condemns NKorea nuke test, calls for UNSC action
Terming the North Korean nuclear test as a "provocative act" in defiance of the will of the international community, the US today asked the United Nations Security Council to take "immediate actions".
"North Korea has claimed it conducted an underground nuclear test. 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," White House spokesman Tony Snow said in a statement.
"We expect the (UN) Security Council to take immediate actions to respond to this unprovoked act," he said.
Snow maintained that Washington is "closely monitoring" the situation and was committed to protect and defend its allies in the region.
President George W Bush was informed of the development shortly before 10pm last night.
US Geological Survey also detected a seismic event in North Korea measuring 4.2 on the Richter scale calling it a "shallow event" or very close to the surface of the earth.
Some analysts meanwhile said the test appeared to be "more of a fizzle than a pop". (Sridhar Krishnaswami for PTI)
Pak distances itself from NKorea nuke programme
Pakistan today distanced itself from North Korea's nuclear programme, saying there was "absolutely no link" between its disgraced scientist A Q Khan, who had confessed to have sold nuclear technology to the Stalinist regime, and the nuclear test conducted by Pyongyang.
"Pakistan deplores the announcement by Democratic Peoples Republic of Korea that it conducted a nuclear test. In our view this is a destabilising development in the region," Foreign Office Spokesperson Tasnim Aslam said.
"We have urged DPRK to desist from introducing nuclear weapons in the Korean peninsula. It is regrettable that DPRK chose to ignore the advise of the international community not to test a nuclear weapons device," she told reporters here.
Aslam, however, was quick to deny links between Pakistan's nuclear programme and that of North Korea, though Khan, currently under house detention here, had publicly confessed in 2003 that he aided North Korean programme.
"There is absolutely no link between the nuclear test conducted by North Korea and what might have gone on (between) Dr A Q Khan and the North Korean government. North Korea's nuclear programme is plutonium-based and Pakistan's programme is mainly Uranium based. There are speculations that the plutonium was diverted from North Korean nuclear facility," she claimed.
President Pervez Musharraf in his memoirs published last month said that Khan had transferred nearly two dozen centrifuges to North Korea. This was in return for Pyongyang providing its missile technology, which reportedly forms the base for Pakistan's nuclear-capable missiles. (K J M Verma for PTI)