North Korea will test a nuclear weapon to deter the "US threat of aggression" and maintain peace on the Korean peninsula, the reclusive communist state announced today, causing alarm across Asia.

South Korea held an emergency Cabinet meeting and raised its security alert. Japan said a nuclear test would be "unforgivable" and the British Foreign Office said the move would be regarded as a "highly provocative act with serious consequences" for North Korea.

The Stalinist regime in Pyongyang, which caused worldwide consternation by test-firing a batch of conventional ballistic missiles in July, announced the plan in a statement to the state-run Korean Central News Agency.

"The DPRK (Democratic People's Republic of Korea) will in the future conduct a nuclear test under the condition where safety is firmly guaranteed," said the Foreign Ministry.

"A people without reliable war deterrent are bound to meet a tragic death and the sovereignty of their country is bound to be wantonly infringed upon."

The statement added that North Korea's overall goal was peace on the Korean peninsula and the removal of nuclear weapons from the region, but the threat of a test is likely to be interpreted as another attempt to provoke America into direct talks with Pyongyang.

The so-called "Six Party" talks — involving the two Koreas, China, Japan, Russia and the US — have been the international community's favoured form of negotiation with the regime and its dictator, Kim Jong Il, but they have been stalled since last November after America targeted a bank in Macau suspected of laundering money for North Korea.

The Bush Administration has yet to react to the statement but John Bolton, the American ambassador to the UN, said this morning that a North Korean nuclear test "would be extraordinarily serious".

Pyongyang wants the US to stop interfering in its financial rackets and hold one-on-one talks, but Washington insists that the country abide by a tentative agreement made at the Six Party talks in September 2005: that North Korea will dismantle its nuclear programme in return for aid and guarantees of its security.

Japan's new Prime Minister, Shinzo Abe, and his conservative Cabinet, responded first to the North Korean statement today, saying: "A nuclear test would be unforgivable for Japan and for the international community."

The Japanese Foreign Minister, Taro Aso, said: "Our response will be severe. This is more serious than the North’s missile tests."

Britain, which joined in the chorus of dismay that greeted North Korea's first missile tests in eight years this summer, said it was monitoring Pyongyang's next steps closely.

"A missile test launch would be viewed by the United Kingdom and the rest of the international community as a highly provocative act with serious consequences for the DPRK," said a Foreign Office spokesman.

"It would raise tensions in an already tense region."

The Russian Foreign Minister, Sergei Lavrov, urged caution on the eccentric communist regime, once a close ally of Moscow, and pointed out that many threats from North Korea have not materialised.

"We are working with Pyongyang in order that it exercises restraint and in order to prevent hasty steps," he said. "It is not the first time that we’ve heard this kind of announcement, but for the most part these announcements have not been confirmed."

North Korea declared itself a nuclear power in February 2005, three years after being named in President Bush's "axis of evil", but has not, so far, shown any evidence of mastering the technology.

Western defence analysts believe that the country, exhausted by years on the brink of famine, has enough fissile material for as many as eight crude nuclear bombs but does not have the capability to mount the material on a missile.

In July, in the midst of severe flooding that killed more than 100 people and displaced thousands, North Korea test-fired seven missiles, all of them medium-range "Scud-type" rockets that it has launched many times before except one: an intercontinental ballistic missile known as a "Taepodong 2" that failed 40 seconds after take-off.

The tests, carried out by Pyongyang in defiance of the advice of its only ally, China, caused alarm in Japan and South Korea and prompted an emergency meeting of the UN Security Council in New York, although little action was eventually taken because the state is already so isolated and weighed down by sanctions.

Chang Myung-Soon, an observer of the North Korean military and a former member of the Korean Institute for National Unification, said Pyongyang had little to lose from the test:

"North Korea thinks it has no other option to press the United States to have bilateral negotiations with them. North Korea has nothing to lose by conducting a nuclear test. It wouldn’t care if its people will starve due to toughened economic sanctions, and a military attack on North Korea will be really difficult considering opposition from South Korea, China and Russia," he told Reuters.