North Korea vowed Wednesday to bolster its nuclear deterrent, describing an exercise by United States and South Korean troops as preparation for a "war of aggression" against the communist state.

The 10-day drill, which began on Tuesday, showed the US was not committed to dialogue over nuclear disarmament, a foreign ministry spokesman said in a statement published by the North's official Korean Central News Agency.

"The prevailing situation goes to prove that the US is not set to realise the denuclearisation of the Korean peninsula through dialogue and negotiations... " he said.

"It is self-evident that the DPRK (North Korea) should put spurs to bolstering its nuclear deterrent for self-defence both in quality and quantity to cope with this situation.

"It is preposterous for the US to urge the DPRK to refrain from bolstering its nuclear deterrent, while whetting its swords for a war of aggression behind the scene of dialogue."

Any aggressive action against the North will face "merciless counteraction", the spokesman added.

During the Ulchi Freedom Guardian exercise US and South Korean troops will simulate the detection and destruction of North Korean atomic bombs, missiles and chemical weapons, according to the Yonhap news agency.

The two countries have described the annual exercise as defensive and routine.

But the North last week urged Washington and Seoul to show their willingness to work towards denuclearisation by scrapping the exercise.

Pyongyang published an open letter in its state media which also called for a peace-keeping mechanism to replace the current armistice that ended the 1950-1953 war.

A flurry of diplomatic efforts have been under way to resume stalled six-party disarmament talks involving the two Koreas, Russia, China, Japan and the United States.

Senior Pyongyang officials met their counterparts in Seoul and Washington last month, raising hopes that the talks -- last held in December 2008 -- could resume.

The North has repeatedly expressed a desire to return to the forum, but the United States has urged it to show more sincerity and mend ties with the South first.