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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“We struck the camp and destroyed it,” he said.

The State Department declared al-Shabaab a terrorist organization in February 2008 and the terror group has launched numerous large-scale attacks in the years since.

Al-Shabaab terrorized the Westgate shopping mall in Nairobi killing 67 in 2013 and the group killed 147 university students in neighboring Kenya in April 2015, singling out Christians during the massacre. That attack occurred at Garissa University College, 200 miles east of the capital city of Nairobi.

A U.S. drone strike in March 2015 killed Adan Garar, an al-Shabaab leader tied to the Westgate mall attack.

In 2014, President Obama announced the head of al-Shabaab, Ahmed Abdi Godane, had been killed in an American drone strike.

The State Department had a $7 million bounty on his head.

Lucas Tomlinson is the Pentagon and State Department producer for Fox News Channel. You can follow him on Twitter: @LucasFoxNews