A missing U.S. military rifle was found Thursday near a local air base, where President Lee Myung-bak departed aboard a presidential jet on the same day for Indonesia to attend a regional summit, an official said Friday.

The U.S. Forces Korea (USFK) official said a civilian found the M4 rifle on Thursday on a road near Seoul Airport, located in Seongnam, just south of the capital city. The discovery apparently sent an alarm to the presidential security detail, but the rifle has since been returned to the U.S. unit, the official added.

"A U.S. soldier at the U.S. 8th Army nearby misplaced the rifle," the official said. "We need to wait and see if there will be further investigation. But it was simply a mistake by the soldier and the case is as good as closed."

On Thursday, Lee embarked on a two-nation trip to Southeast Asia for a series of regional cooperation talks in Indonesia and a state visit to the Philippines.

It has not been known whether the gun was discovered before or after the takeoff of the presidential jet.

South Korea's Air Force One is stationed at Seoul Airport, and heads of state and other guests land there on their visits to South Korea. (Yonhap News)