A U.S. Marine helicopter crashed Friday in a rice paddy during joint exercises with South Korean forces and killed all four crewmen aboard, the U.S. military said.

The CH-46 helicopter went down near Toksok-Ri on the east coast while taking part in annual "Team Spirit" exercises, a U.S. military statement said. No one was reported injured on the ground.The statement said the cause of the crash was unknown.

In Washington, the Marine Corps identified the dead as Capt. Lawrence L. Beason Jr., 28, of Plano, Texas; 1st Lt. Christopher M. O'Connor, 26, of Gaylord, Mich.; Cpl. Michael W. Vinet, 22, of Riverside, Calif., and Lance Cpl. Jon D. Edwards, 19, of Poway, Calif.

The helicopter was assigned to the 13th Marine Expeditionary Unit based at Camp Pendleton, Calif., the statement said. The troop-transport helicopter had been operating from the Marine assault ship the USS Belleau Wood.

About 200,000 U.S. and South Korean troops are taking part in the maneuvers to practice bringing in U.S. reinforcements in the event of an attack by communist North Korea.

The United States has about 42,000 troops based in South Korea under a mutual defense treaty.