More than 150 al-Shabaab fighters were killed as they stood in formation at a graduation ceremony, a U.S. official said Monday.

The strike, which used manned and unmanned aircraft, targeted a terrorist training camp in Raso, Somalia. Raso is located 120 miles north of the capital city of Mogadishu.

The airstrike was done in “self-defense,” Air Force Secretary Deborah James said at a news conference.

The camp, which held up to 200 fighters at a time, was preparing to conduct a “large scale attack” against U.S. and African forces sometime in the future, according to Pentagon spokesman Capt. Jeff Davis.