A spokesman for the Committee for the Peaceful Reunification of the Fatherland warned it had already built an "invincible deterrent" to frustrate the plan, the Korean Central News Agency said.

The spokesman denounced the contingency plan, codenamed OPLAN 5029-05, as showing the United States was intent on toppling the communist regime despite denials of any intention to attack North Korea.

"The DPRK (North Korea) having powerful armed forces for self-defence...is fully prepared to mercilessly beat back any armed provocation of the US and its allied forces," the spokesman said in a statement carried by the agency.

"If the US and the South Korean warmongers ignite a war..., the army and people of the DPRK will... wipe out the aggressors with the invincible deterrent force built up for scores of years."

Seoul's National Security Council said last week without elaborating that it had vetoed the classified US-South Korean combined forces plan because it could infringe on South Korean sovereignty.

Under a mutual defense treaty, the South Korean military comes under US command only in times of war.

Analysts say the highly confidential plan would allow the US military to easily take over from South Korean forces to handle massive disruption envisaged by the potential collapse of impoverished North Korea.

The goal of the top secret military operation would be to secure North Korea's nuclear weapons sites and materials, they say.

North Korea has been locked in a standoff with the outside world for more than two years over its nuclear weapons drive.

Washington believes North Korea has one or two crude nuclear bombs and has reprocessed enough plutonium for several more.

South Korea reportedly has its own contingency plan on how to react to a sudden collapse of North Korea.

Under the plan revealed in a newspaper report in October last year, South Korea would move swiftly to take control of its communist neighbor, installing a top Seoul official as governor and opening camps for 200,000 refugees.

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