The United States and South Korea have begun a robust week-long series of annual military exercises aimed at ensuring the two allies can deter and defeat any incursion by North Korea. As it has often done in the past, Pyongyang is condemning the exercises, calling them a rehearsal for aggression against the North.

Sunday's drill mobilization is similar to other past exercises held on an annual basis, aimed at simulating a combined U.S. - South Korean response to a North Korean attack.

U.S. and South Korean military officials say about 20,000 U.S. personnel are taking part, as well as a significant portion of South Korea's 680,000 military personnel. They are supported for the first time by the aircraft carrier U.S.S. Nimitz and several other U.S. warships docked near the southern port city of Busan.

North Korea invaded South Korea in 1950. An armistice was signed three years later, but there has never been a formal peace treaty to conclude the conflict. Washington stations about 28,000 forces here to help enforce the armistice, and deter a repeat of the North's invasion.

The week's exercises include field drills and computer simulations of the movement of personnel and equipment. South Korean Colonel Kim Sang-do says such drills help ensure the integrity of crucial supply lines in a potential battle scenario.

Kim says the exercises will enhance the joint military capability of the U.S. and South Korea.

As it has often done in the past, North Korea is issuing harsh public criticisms of the exercise. The North's main news agency quotes an army spokesman who describes the drills as "madcap war exercises." A government newspaper in Pyongyang calls them "reckless maneuvers for a war of aggression" against North Korea.

U.S. and South Korean authorities insist the exercises are purely defensive in nature, and say they have notified the relevant North Korean authorities of this week's training schedule. President Bush and other senior U.S. officials have said repeatedly the United States does not intend to attack North Korea.

However, the exercises coincide with a difficult period in multinational negotiations aimed at ending North Korea's nuclear weapons programs. Pyongyang promised it would declare all of its nuclear activities by the end of 2007, but has still failed to do so.

Senior U.S. negotiator in those talks, Assistant Secretary of State Christopher Hill, was expected to leave Beijing Sunday after failing to meet with his North Korean counterpart. Hill was quoted as saying the North Koreans are "not ready" to discuss pushing the nuclear talks forward.

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