Tens of thousands of US and South Korean troops are taking part in the annual military exercises, which North Korea has branded "test nuclear war maneuvers."

"We have an important alliance with South Korea. We have done these exercises before," State Department spokesman Philip Crowley told reporters.

"These should not be a surprise to North Korea," he said.

North Korea, which habitually protests war games in the democratic South, put its 1.2 million-strong military on alert and said it was abandoning efforts toward nuclear disarmament.

The communist state has been under heavy international pressure to return to six-nation talks on ending its nuclear program.

North Korea, which has tested two atom bombs, had earlier agreed in general terms to return to six-way talks but has pushed first for negotiations with the United States on a permanent peace treaty.

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