A U.S. drone strike has killed more than 150 al-Shabaab Islamist militants in Somalia who were in the final stages of preparing to attack U.S. troops and African peacekeeping forces, according to the Pentagon, CNN reported.

The death toll is the largest in a U.S. military attack in the post-9/11 era, according to the Guardian.

The Saturday strike used drones and manned aircraft to target al-Shabaab’s Raso training camp 120 miles north of the capital Mogadishu.

The U.S. military monitored the camp for weeks, gathering intelligence before the strike, the Pentagon said on Monday.

Al-Shabaab is responsible for terror attacks on Kenya’s Westgate shopping mall and Garissa University in Kenya that killed hundreds of shoppers and students, ABCNews reported.

In recent years, the U.S. military has carried out airstrikes and special operations against al-Shabaab leaders. In Somalia, al-Shabaab focuses its attacks against African Union targets.

“On Saturday, March 5, the U.S. military, in self defense and in defense of our African Union Mission in Somalia (AMISOM) partners, conducted an airstrike in Somalia against Raso Camp, a training facility of al-Shabaab, which is a terrorist group affiliated with al-Qaeda,” said Pentagon Press Secretary Peter Cook.

What exactly the imminent threat was, the Pentagon isn’t saying, CNN reported. “Drones carry precision missiles that go against pinpoint targets. Training camps tend to be dispersed — personnel out in the open, moving around,” said a CNN commentator. “It’s not at all clear how they used drones to kill 150 people at a dispersed training camp.”

African Union peacekeeping forces chased al-Shabaab out of Mogadishu in 2011 but it has remained a threat in Somalia, making frequent attacks to try and overthrow the Western-backed government, the Guardian reported.

Its name means “The Youth,” and it wants to impose a strict version of sharia law in Somalia, where it targets government and security interests, hotels and restaurants in the capital.

Al-Shabaab was behind deadly attacks in Uganda and Kenya, which both contribute troops to Somalia’s African Union peacekeeping force.

Up to 200 fighters were at the Raso camp at the time of the strike. No civilian casualties are reported, and no U.S. ground forces were used in the strike.

The strike will hamper al-Shabaab’s ability to recruit new members, establish bases and plan attacks on U.S. and Amisom forces in Somalia, Davis said.