South Korea and the United States began joint military exercises on Saturday amid angry protests by North Korea, which has denounced the annual exercises as preparations for a pre-emptive nuclear attack.

About 25,000 U.S. troops and an undisclosed number of South Korean soldiers will participate in the weeklong exercises, which involve a computer-simulated war game and field exercises aimed at improving U.S. and South Korean forces' defense capabilities, according to the U.S. military command.

"The purpose of the drill is defensive," said Kim Yong-kyu, a spokesman for the U.S. military command in Seoul, dismissing as "nonsense" North Korea's claims that the military exercises are preparations to invade the communist state.

North Korea has stepped up its anti-U.S. rhetoric over the exercises, vowing to take an unspecified strong measure of self-defense and suggesting it had the ability to launch a pre-emptive attack on the United States.

The drills are being held out of view of any spectators.

About 29,500 U.S. troops are stationed in South Korea as a legacy of the 1950-53 Korean War that ended in a cease-fire, not a peace treaty, leaving the two Koreas technically at war. The number of American troops is set to decline to 25,000 by 2008 as part of the Pentagon's worldwide realignment of its forces.