The Pentagon on Monday said an American aircraft had hit a training camp of the militant group al-Shabaab, an al Qaeda affiliate in Somalia, killing 150 fighters. American officials believe that Saturday’s strike caught the fighters while they were at a graduation ceremony ahead of a planned attack on American troops and their allies in East Africa, The New York Times reported. The announcement of the strike was delayed so United States officials could assess its outcome, they said.

Officers confirmed that they carried out the strike through drones and precision-guided bombs and missiles. They also claimed there were no civilian casualties, though this has not been independently verified.

This is the US’s strongest attack against al-Shabaab, as it has only targeted its leaders and never its junior fighters before. The group was behind the attack on Westgate shopping mall in Nairobi in 2013, one of the continent’s worst terror tragedies.