SEOUL, South Korea (AP) — A high school principal at the U.S. Osan Air Base in South Korea said Monday that officials locked down the school and base as a precaution after someone reported an unscheduled "active shooter drill."

Morgan Nugent, principal of Osan American High School, said in a posting on the base's official Facebook site that officials "decided to error on the side of safety" and lock the school down after a teacher received what sounded like an automated call saying there was "an active shooter drill taking place." There was no such drill scheduled Monday.

Nugent said security forces were sweeping the school and its perimeter and he would restart school once given an "all clear."

People on the base had been told to stay in their quarters or workplaces until further notice, according to Tech. Sgt. Stacy Foster, a spokesman at Osan. He had no other details.

Osan Air Base is located south of Seoul. About 28,500 U.S. troops are stationed in South Korea as a deterrent to North Korea, whose 1950 attack started the Korean War.

Osan is the headquarters of the U.S. air forces in South Korea, the 7th Air Force. The base is also home to the 7th Air Forces' 51st Fighter Wing. About 7,500 to 8,000 U.S. troops, civilian employees and their family members live and work on base, according to the base's public affairs office.

The presence of the U.S. troops and annual U.S.-South Korean war drills are a continuing point of contention with North Korea, which claims that they are signs of hostility and designs at invading the North.