The order from the Supreme Command, whose supreme commander is North Korean leader Kim Jong Il, suggests an unusually tough stance on the Ulji Freedom Guardian joint military exercises, which run through Aug. 27.

North Korea will retaliate against any military provocation with "a merciless and prompt annihilating strike at the aggressors with all offensive and defensive means including nuclear deterrent involved," the KPA's top command was quoted by KCNA as saying.

Along with all military forces, the top command ordered militia groups such as the Worker-Peasant Red Guards and the Young Red Guards to "closely follow all the moves" by the U.S. and South Korean forces with "high vigilance" and "make an immediate strong attack on perpetrators of any hostile act" against North Korea.

North Korea calls the U.S.-South Korea drill a prelude to an invasion, but Washington and Seoul refute such a claim, saying it is purely defensive.

The top command urged all nationals to "bring about a new leaping forward" in North Korea's "150-day campaign" to step up the building of a prosperous and powerful socialist nation, KCNA said.

In the campaign, which will run through Sept. 16, North Korea plans to lift the economy by developing four key sectors -- power generation, coal, steel and metal, and railway transport.

The two Koreas are still technically at war because the Korean War ended in an armistice rather than a permanent peace treaty. They keep millions of troops deployed on both sides of the border.