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.

The attack occurred “near the end of that training,” Davis said, but he did not reveal how the U.S. military made that determination.

There were no indications of civilian casualties, Davis said.

The camp had been under close observation for two weeks before the strike, according to Davis.

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