MOGADISHU, Somalia — U.S. military forces attacked the extremist al-Shabab network in Somalia Monday, the Pentagon said, and a witness described ground-shaking explosions in a strike that reportedly targeted the group’s leader.

Al-Shabab had attacked the upscale Westgate Mall in Nairobi, Kenya, killing at least 67 people a year ago this month and the U.S. had targeted planners of the bloody assault. There was no immediate comment from al-Shabab and U.S. commanders were waiting to determine the attack’s outcome.

“U.S. military forces conducted an operation in Somalia today against the al-Shabab network. We are assessing the results of the operation and will provide additional information as and when appropriate,” said Pentagon Press Secretary Rear Adm. John Kirby.

After the U.S. strike Monday night in a forest south of Mogadishu, masked Islamic militants in the area arrested dozens of residents they suspected of spying for the U.S. and searched nearby homes, a resident said.

“Mass arrests just started, everyone is being detained,” said Mohamed Ali, who lives in Sablale district. “They even searched nearby jungles and stopped the nomads transporting milk and grass to the towns for questioning.”