North Korea vowed on Monday to strengthen its defences amid concerns the country may conduct a nuclear test as a follow-up to last month's long-range rocket launch.
Citing US hostility, Pyongyang's foreign ministry said in a memorandum that North Korea will "continue to strengthen its deterrence against all forms of war".
The memorandum carried by state media did not say what action North Korea would take to defend itself. However, North Korea has claimed the right to build atomic weapons to protect itself from the United States, which stations more than 28,000 troops in South Korea.
It came as reports said North Korea has informed China that it is planning to conduct the nuclear test this week.
Quoting South Korean intelligence sources, local media said Pyongyang warned its main political ally that the underground test would be carried out before January 20.
Since North Korea successfully launched a ballistic missile in December, there has been growing concern that the regime of Kim Jong-un would follow that triumph up with its third nuclear test.
The intention, analysts believe, is to demonstrate that it has the technology both to build a nuclear warhead and the missile system to deliver it to a target.
"We have heard that a North Korean official in Beijing told the Chinese side that the North planned to carry out a nuclear test between January 13 and 20," the South Korean Joongang newspaper quoted an official in Seoul as saying.
"We are now stepping up surveillance of the Punggye-ri nuclear test site," he said.
North Korea has previously conducted two underground nuclear tests at the site, in 2006 and again in 2009, in defiance of international pressure and sanctions.
Satellite images of the Punggye-re site taken before the North conducted its missile test on December 12 showed renewed activity, including excavation equipment used to dig the tunnel for the test warhead and debris piles.
Yu Woo-ik, South Korea's Unification Minister, told a parliamentary committee in December that it was "highly probable" that Pyongyang is planning to follow up the successful rocket launch with another nuclear test.
"Judging from analysis of intelligence, significant preparations have been made," he said.
Members of the United Nations Security Council are presently discussing new sanctions against Pyongyang as a result of the missile test, although Beijing is resisting demands for swingeing sanctions to be imposed on its ally.
Yang Min-jin, a professor the University of North Korean Studies in Seoul, played down suggestions that Pyongyang might be planning a nuclear experiment.
"Chances are slim that the North might push ahead with a nuclear test in the winter season, especially when China is insisting on a moderated response to the rocket launch to prevent a third nuclear test taking place," he told the Joongang daily.
North Korea's foreign ministry also urged Washington to dismantle the US-led UN Command, which oversees an armistice signed at the close of the Korean War in 1953. It accused the US of trying to turn the UN Command into a Nato-like regional military bloc.
"Whether the US immediately dismantles the UN Command or not will serve as the acid stone in deciding whether the US will maintain or not its anti-(North Korea) hostile policy," said the memorandum, which was carried by the official Korean Central News Agency.
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