Hundreds of U.S. Marines stormed a South Korean beach Friday, carrying out war games that North Korea charges are a prelude to an invasion.

The maneuvers, supported by fighter jets from warships just off shore, came even as other Marines, U.S. troops and allies crossed into Iraq in a march to disarm Saddam Hussein.

Both South Korean and U.S. Marines came ashore on amphibious assault vehicles amid mock explosions. Waves of helicopters and fighter jets droned overhead during the exercises, part of ongoing annual maneuvers.

After jumping out of the landing craft, the troops scrambled up an embankment to take positions behind pine trees before spreading out across the nearby rice fields.

"This is not about North Korea. This is about our commitment to deterrence. It is a defense-oriented exercise," said Lt. Col. Mike Caldwell, a spokesman for the U.S. Forces in Korea.

A lone demonstrator ran onto the beach to unfurl a protest flag before the drills began near this seashore village in southeastern South Korea, but was quickly tackled by police and hauled away.

The drills come amid heightened tensions over North Korea's suspected nuclear weapons program. But the United States, which bases 37,000 troops in South Korea, says they are unrelated to the standoff and purely defensive.

Washington has deployed an intimidating array of weaponry for the monthlong maneuvers, including the aircraft carrier USS Carl Vinson and a wing of radar-evading stealth fighters, which is here for the first time in a decade.

The United States regularly conducts military exercises with South Korea. One of the annual exercises -- called "Reception, Staging, Onward Movement and Integration" -- will end March 26. A second exercise -- called "Foal Eagle" -- started March 4 and will run until April 2.

North Korea criticizes the drills as unnecessarily stoking jitters despite Washington's repeated insistence it has no plans to invade North Korea.

Tensions have run high in the region since October, when the United States said North Korea admitted having a secret nuclear program.