Feisal Omar / Reuters A Somalia government soldier patrols on a vehicle in the area south east of Dusamareeb, March 19, 2014.

WASHINGTON (Reuters) - The United States has carried out an air strike in Somalia, killing more than 150 fighters with the al Qaeda-linked Islamist group al-Shabab, following U.S. intelligence on preparations for a large-scale militant attack, the Pentagon said on Monday.

The weekend strike using both manned and unmanned drone aircraft targeted al-Shabab's "Raso" training camp, a facility about 120 miles north of the capital Mogadishu, the Pentagon said.

The U.S. military said it had been monitoring the camp for several weeks before the strike and had gathered intelligence, including about an imminent threat posed by the trainees.

"We know they were going to be departing the camp and that they posed an imminent threat to U.S. and to Amisom, African Union mission in Somalia forces, that are in Somalia," said Pentagon spokesman Captain Jeff Davis.

Davis declined to disclose any specific information the United States might have about the group's intended target.

The al Qaeda-linked al-Shabab was pushed out of Mogadishu by African Union peacekeeping forces in 2011 but has remained a potent antagonist in Somalia, launching frequent attacks in its bid to overthrow the Western-backed government.

The group, whose name means "The Youth," seeks to impose its strict version of sharia law inSomalia, where it frequently unleashes attacks targeting security and government targets, as well as hotels and restaurants in the capital.

Al-Shabab was also behind deadly attacks in Kenya and Uganda, which both contribute troops to an African Union peacekeeping force in Somalia.

Davis said as many as 200 fighters were believed to be training at the Raso camp at the time of the strike and expressed confidence there were no civilian casualties.

"Their removal will degrade al-Shabab's ability to meet the group's objectives in Somalia, which include recruiting new members, establishing bases and planning attacks on U.S. and Amisom forces there," Davis said.

No U.S. forces on the ground participated in the strike.

"It was an air operation," Davis said.