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-Shabaab 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.”

A senior Somali intelligence official said a U.S. drone targeted al-Shabab leader Ahmed Abdi Godane as he left a meeting of the group’s top leaders. Godane, also known as Mukhtar Abu Zubeyr, is the group’s spiritual leader under whose direction the Somali militants forged an alliance with al-Qaida.

The Somali official, speaking on condition of anonymity since he was not authorized to speak to the media, said intelligence indicated Godane “might have been killed along with other militants.” The official said the attack took place in a forest near Sablale district, 105 miles (170 kilometers) south of Mogadishu, where al-Shabab trains its fighters.

The governor of Somalia’s Lower Shabelle region, Abdiqadir Mohamed Nor, told The Associated Press that as government and African Union forces were heading to a town in Sablale district, they heard what sounded like an “earthquake” as the al-Shabab bases were hit.

“There was an airstrike near Sablale. We saw something,” Nor said.

The U.S. has carried out several airstrikes in Somalia in recent years.

A U.S. missile strike in January killed a high-ranking intelligence officer for al-Shabab, and last October a vehicle carrying senior members of the group was hit in a U.S. strike that killed al-Shabab’s top explosives expert.

The latest U.S. action comes after Somalia’s government forces regained control of a high-security prison in the capital that was attacked on Sunday. Seven heavily armed suspected al-Shabab members had attempted to free other extremists held there.

Somali officials said all seven attackers, three government soldiers and two civilians were killed. Mogadishu’s Godka Jilacow prison is an interrogation center for Somalia’s intelligence agency, and many suspected militants are believed to be held in underground cells there. The attack started when a suicide car bomber detonated an explosives-laden vehicle at the gate of the prison and the gunmen then fought their way into the prison.

Al-Shabab had attacked the mall in Nairobi last year in retaliation against Kenya for sending troops into Somalia against the extremists. Godane said at the time that the attack was carried out in retaliation for the West’s support for Kenya’s Somalia intervention and the “interest of their oil companies.”

Al Shabab is now mostly active in Somalia’s rural regions after being ousted from the capital by African Union forces in 2011.

Somali military officials last week launched a military operation to oust al-Shabab from its last remaining bases in the southern parts of Somalia. On Saturday the militants withdrew from the town of Bulomarer, located about 110 kilometers (70 miles) south of Mogadishu, after hours of fighting.