South Korean army soldiers cross floating bridge on  Han River during annual military exercise against possible attack from North

South Korean Defense Authorities say they are rejecting a contingency plan that would give command authority to the United States military in the event of a North Korean collapse.

South Korea's National Security Council on Friday said it had vetoed a joint military plan with the United States on how to handle serious turmoil in North Korea, should it arise.

The United States had asked that the plan be approved last year. It would have given the United States command over South Korean military assets in the event of rioting, mass defections or a government collapse in the impoverished North.

U.S. officials reportedly had argued that the contingency plan was necessary to secure sensitive nuclear and military facilities, and for overall public safety. However, South Korean officials said they were dropping the plan because it could limit "South Korea's exercise of its sovereignty."

Timothy Savage, a researcher with the International Crisis Group in Seoul, says the way the announcement was made is likely to irritate the United States.

"These are the kind of things that would normally come out of alliance negotiations," he said. "But when the South Korean NSA is making unilateral announcements like this, they're clearly not consulting with the Americans beforehand."

South Korean President Roh Moo-hyun is pursuing a policy of greater independence from its Cold War alliance with the United States. His government plans to increase military cooperation with China, and for South Korea to become what he calls a "balancing power" in Asia.

Washington has been a South Korean ally since the peninsula was divided at the end of World War II. The United States led the United Nations force that supported South Korea in its war against the communist North in the 1950s, and U.S. troops have remained in the South since. Under their alliance, should hostilities resume with North Korea, the United States would have overall command of both its own and South Korean forces.

The two Koreas technically remain at war, because no peace treaty was signed at the end of the war in 1953.

However, in the past year, the United States has begun to reduce its force in South Korea - now around 32,000 troops.

Some security analysts say that Friday's announcement is simply the latest sign of strain in the alliance.

One significant strain has been their different approaches in dealing with North Korea, which has declared that it has nuclear weapons. Washington distrusts the North and wants it to give up its nuclear weapons, or risk further isolation. Seoul, however, concerned about a possible collapse of its neighbor, has a policy of engaging with Pyongyang, in the hope of encouraging peaceful reforms in the Stalinist state.