The official KCNA news agency said the test was successfully conducted at 10:36am (0136 GMT).

"The nuclear test was conducted by 100 percent of our wisdom and technology," it said, claiming there was no danger of radioactive leaks.

"Our science and research centre safely and successfully conducted an underground nuclear test on October 9, 2006."

South Korea's intelligence agency detected a 3.58 magnitude seismic tremor at the time of the test, a foreign ministry spokesman in Seoul said.

South Korean President Roh Moo-hyun immediately called an emergency meeting of security officals because of what the foreign ministry said was "a grave change in the situation involving the North's nuclear activity."

News of the test came as Japanese Prime Minister Shinzo Abe landed in Seoul seeking its support for a common line against the reclusive North.

"President Roh Moo-hyun called in an emergency meeting of related ministers to discuss the North Korean nuclear issue," South Korea's Yonhap news agency quoted foreign ministry spokesman Choo Kyu-Ho as saying.

"The government has received a report that there was a tremor of 3.58 sensed from North Korea's northern Hamkyong province at 10:36 am (0136 GMT)," Choo Kyu-Ho told AFP.

Another ranking foreign ministry official quoted by Yonhap earlier said the government had received intelligence that North Korea appeared to have already carried out a nuclear test.

"We are not in a position to confirm such a report,' a South Korean defence ministry spokesman told AFP.

Choo said that security-related ministers had been holding talks at the presidential office since 11:30 am (0230 GMT) to discuss whether it was related to a nuclear test.

Pyongyang had announced on October 3 through its foreign ministry that it planned a test under safe conditions, citing "the US daily increasing threat of a nuclear war and its vicious sanctions and pressure."

That announcement triggered worldwide alarm and appeals for a change of heart, including a UN Security Council statement.

The United States and South Korea warned they could not tolerate a nuclear-armed North, Japan said any test would be unforgivable and China, Pyongyang's main ally, urged it to show restraint.

The world's eighth declared nuclear power is a secretive and volative regime which for five decades has been technically at war with South Korea and the United States.

It is believed to have produced enough weapons-grade plutonium to make several crude nuclear bombs, according to US and South Korean experts.

The North, which pledges no first use of any nuclear weapons, also has an advanced missile programme although it is not known whether it could arm one with a nuclear warhead.

On July 5, it test-fired seven missiles which fell into the sea - including a Taepodong-2 believed to be technically capable of hitting the United States.

In its October 3 statement, the North pledged never to use nuclear weapons first and strictly to ban the transfer of nuclear weapons and technology.

"The ... nuclear weapons will serve as reliable war deterrent for protecting the supreme interests of the state and the security of the Korean nation from the US threat of aggression and averting a new war..." it said at the time.

North Korea had kept the world guessing for almost 20 years about its progress towards a bomb, declaring only in February 2005 that it possessed nuclear weapons.

The United States had said it would press for mandatory UN sanctions after any test, but played down the prospect of a military response -- something Seoul has also ruled out.

Analysts had said a test may trigger a regional nuclear arms race, and Leave international non-proliferation efforts in tatters.

Agence France-Presse