The annual U.S.-South Korean military maneuvers, involving about 200,000 personnel, began Monday, the Combined Forces Command said.
The "Team Spirit '86" exercise, the 11th in the series, will continue until late April. About 70% of the troops involved are South Korean and 30% American, officials said.
No details were given, but the exercise in the past has consisted of deployment of personnel and equipment during the first few weeks, leading to later joint operations.
In addition to participation of many of the 39,000 U.S. troops based in South Korea, other American military personnel were to arrive from several Pacific bases, Hawaii and the United States.
When plans for the exercise were announced Jan. 18, North Korea reacted by calling off scheduled contacts with South Korea on economics, Red Cross humanitarian issues and parliamentary exchanges.