The country's National Security Council said it had ordered the classified plan to be scrapped because it could infringe on South Korean sovereignty.

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

Analysts said the US military may have wanted control of South Korean forces to handle massive disruption envisaged by the potential collapse of impoverished North Korea which has been in a standoff with the outside world for more than two years over its nuclear weapons drive.

The goal of the top secret military operation, codenamed 5029, would be to secure North Korea's nuclear weapons sites and materials, they said.

In a statement from the nation's top decision-making body on security matters, the NSC said it had killed off the plan earlier this year.

"After receiving a report in December 2004 from the (South Korean) joint chiefs of staff that the combined forces command has been working on Operation plan 5029, the NSC, along with related government agencies, studied its contents," the statement said.

"After reviewing it, the NSC determined that some points in the plan could serve as factors limiting South Korea's exercise of its sovereignty...

"In January 2005, the NSC's standing committee concluded that it is necessary to stop the promotion of the operation plan. The defense ministry, then, notified the Combined Forces Command of its decision."

The US military declined to discuss the statement issued by the NSC and said it could not comment on classified matters.

"As a matter of policy, the United States does not discuss matters relating to operational plans, or the existence or non-existence of particular operational plans," it said in a statement.

"All questions regarding policy statements or positions from the ROK (Republic of Korea) National Security Council should be addressed directly to them.

The statement from the NSC was the latest sign of a strain in relations between the South Korean government and the United States.

South Korean President Roh Moo-Hyun, elected on a wave of anti-American sentiment in December 2002, has stressed the importance of the alliance dating back more than five decades.

But differences over how to handle North Korea have caused friction at the same time as Seoul's ties with Tokyo -- Washington's other key regional ally -- have turned sour over a sovereignty dispute over small islands in the sea between the two counties

Recently Roh has been pushing for a more neutral role for South Korea in the region, talking of a regional "balancer role" suggesting a warming to China and an easing away from the Washington-Tokyo axis.

For Washington, the main concern in the event of instability in North Korea is to ensure that nuclear materials are secured, experts said.

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

South Korea's own secret plans on how to react to a sudden collapse of the North were revealed in October.

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

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