North Korea Monday urged South Korea and the United States to cancel their upcoming annual joint exercise if they want to see a thaw in relations as well as denuclearisation.

The exercise is preparation for "a war of aggression" against the North and will involve "nuclear war manoeuvres", its military said in an open letter published by the official news agency.

The US and South Korea "should show their willingness to denuclearise the peninsula in the eyes of the world by cancelling the projected nuclear war exercises", it said.

The 10-day Ulchi Freedom Guardian exercise from August 16 is an annual computer-assisted simulation command-post exercise.

US and South Korean officials described it as defensive and routine but the North habitually terms such joint drills a rehearsal for invasion.

The letter said Seoul and Washington should show their willingness for denuclearisation by making "a bold policy decision" to cancel the exercise.

Denuclearisation includes the removal of nuclear threats to North Korea, it said, warning that the US and South Korea would not remain safe in case of a new war.

"If the other party launches a nuclear war, the DPRK (North Korea) is ready to counter it with nukes. It is its solemn declaration."

The letter comes amid a flurry of diplomatic efforts 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 for the resumption of the forum last held in December 2008.

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

In its letter, the North called on the US and South Korea to make a "bold practical decision" to replace the current armistice with a peace-keeping mechanism.

The armistice ended the 1950-1953 war but a peace treaty has never been signed.