"The security forces sweeps are now complete.There were no reported injuries or suspicious activity found," the 51st Fighter Wing stationed at Osan Air Base said in a post on its official Facebook page.
"Everyone is safe. The lockdown has been lifted," it added. Osan Air Base, some 65 miles south of Seoul, is one of a number of facilities housing the 28,500 US military personnel permanently based in South Korea.
The facility went into lockdown shortly after 11:00 am (0200 GMT) as a precautionary measure following what high school principal Morgan Nugent suggested was a possible hoax call.
Everyone is safe. The lockdown has been lifted: officials
"One of our teachers received a call that sounded automated saying that there was an active shooter drill taking place," Nugent said. "There was no scheduled school shooter drill on base, so we have decided to err on the side of safety and went into a full school lockdown," he added.
The safety measure was extended to the entire base as security forces swept the high school, middle school and elementary school buildings and grounds.