North Korea on Saturday threatened a "sacred war" over US-South Korean joint military drills next week as a US envoy urged Pyongyang's new leadership to engage in dialogue with the outside world.

The North's National Defence Commission (NDC) denounced the annual joint exercises as a "silent declaration of war," describing them as "unpardonable war hysteria."

Key Resolve, a computerised command post exercise, will start on Monday and continue until March 9. Separately, the joint air, ground and naval field training exercise Foal Eagle will be held from March 1 to April 30.

"Key Resolve and Foal Eagle are unpardonable war hysteria kicked up by the hooligans to desecrate our mourning period and an unpardonable infringement upon our sovereignty and dignity," the NDC said in a statement.

"Our army and people will foil the moves of the group of traitors to the nation and warmongers at home and abroad for a new war with a sacred war of our own style," the NDC said, indicating it would stage a counter exercise.

The threat is the latest instance of Pyongyang taking a hostile tone towards Seoul since Kim Jong-Un, the youngest son of the late leader Kim Jong-Il, took over after his father Kim Jong-Il died of a heart attack on December 17.

Last week the North vowed "merciless retaliatory strikes" if any shells landed in waters claimed by Pyongyang during a live-fire artillery exercise near the disputed Yellow Sea border.

But in the event it took no military action in response to the drill.

"War manoeuvres... are, in essence, a silent declaration of a war. The declaration of the war is bound to be accompanied by a corresponding physical retaliation," the NDC said Saturday.

"Now that a war has been declared against us, the army and people are firmly determined to counter it with a sacred war of our own style and protect the security of the nation and the peace of the country," it added.

It accused "US imperialists" of intensifying moves to ignite a war, and implied that the North had the capacity to strike against the US mainland.

"Nuclear weapons are not the monopoly of the US. We have war means more powerful than the US nukes and ultra-modern striking equipment which no one has ever possessed," it said.

"The US is sadly mistaken if it thinks it is safe as its mainland is far away across the ocean. There is no limit to the striking intensity and range of our army and people to wipe out the aggressors."

Professor Kim Yong-Hyun of Dongguk University in Seoul said the North's statement was aimed at increasing pressure on the US and South Korea over the exercise, while at the same time seeking solidarity among its own people.

"However, it is unlikely for the North to commit militarily provocative actions as it needs to push forward with dialogue with the United States," he told AFP.

US and North Korean officials this month resumed negotiations aimed at reopening six-party talks on dismantling the North's nuclear programme in return for diplomatic and economic gains.

Pyongyang walked out of the process -- which is chaired by China and also brings together the two Koreas, the US, Russia and Japan -- in 2009 before carrying out a second nuclear test, but now wants to return, albeit without preconditions.

Glyn Davies, coordinator for United States policy on North Korea, said in Seoul it was "significant" that the North decided to reengage in talks "in a relatively short period of time after the change of leadership" in the North.

"We hope and we expect that the DPRK (North Korea) will choose to go down the path for a greater engagement and indeed ultimately a greater cooperation," he told journalists after briefing South Koreans on two days of discussions with North Korean diplomats in Beijing.

Earlier in Seoul, he said there was still a "long way" to go before the six-party talks could resume.

Professor Yang Moo-Jin of the University of North Korean Studies said Pyongyang was seeking to add an element of urgency to the discussions about the six-party talks.

"Basically, it is telling the United States, 'You'd better not miss this opportunity'. Otherwise, we will further build up our nuclear deterrence including ICBMs," Yang said, referring to intercontinental ballistic missiles.