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WASHINGTON — More than 150 members of the al-Shabab terrorist group were killed overnight Sunday by a massive U.S. airstrike in Somalia, senior defense officials told NBC News.

The dead were among the 200 or so fighters at a training camp some 120 miles north of Mogadishu who were preparing for an "imminent large-scale attack" against U.S. and regional governmental forces, officials told NBC News.

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While the officials did not identify the al-Shabab targets, American special forces have been training Somali forces in the east African nation.

It appeared "the group was in formation" and conducting "some kind of ceremony" when American drones dropped bombs and launched Hellfire missiles at the militants, the officials said.

It was not clear whether any senior al-Shabab leaders were killed in the strikes.

Related: Al-Shabab Claims Responsibility for Somalia Plane Bomb

Al-Shabab has been responsible for terrorist attacks throughout East Africa, including the massacre of 67 civilians during a four-day siege at the Westgate Mall in Nairobi, Kenya, in September 2013.

Similar U.S. airstrikes in the region have been previously launched from an American airbase in Djibouti in the Horn of Africa.