Al-Shabab's top leader was travelling in one of two vehicles hit Monday night by a U.S. military strike, a member of the Somali Islamic extremist group said Tuesday.

The spokesman would not say whether Somali Ahmed Abdi Godane, the rebel leader, was among the six militants killed.

The two vehicles were heading toward the coastal town of Barawe, al-Shabab's main base, when they were hit, Abu Mohammed told The Associated Press.

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U.S. military forces attacked the extremist al-Shabab network in Somalia Monday, the Pentagon confirmed.

A witness in Somalia described ground-shaking explosions caused by the strike. Somali government and African Union forces heading to a town in the district heard what sounded like an "earthquake" as the al-Shabab bases were hit, the governor of Somalia's Lower Shabelle region, Abdiqadir Mohamed Nor, told The Associated Press.

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

Al-Shabab attacked the upscale Westgate Mall in Nairobi, Kenya, killing at least 67 people a year ago this month and the U.S. targeted planners of the bloody assault.

The U.S. drone strike targeted Godane as he left a meeting of the group's top leaders, said a senior Somali intelligence official. Intelligence indicated Godane "might have been killed along with other militants," said the Somali official, speaking on condition of anonymity since he was not authorized to speak to the press.

The attack took place 170 kilometres south of Mogadishu, where al-Shabab trains its fighters, he said.

Godane, also known as Mukhtar Abu Zubeyr, is al-Shabab's spiritual leader under whose direction the Somali militants forged an alliance with al-Qaeda. In 2012 the U.S. offered a reward of up to $7 million for information leading to his arrest.

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U.S. commanders said they are waiting to determine the outcome of Monday's attack.

"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 in a forest near Sablale district 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."

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.

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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 centre 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 attacked the mall in Nairobi last year to punish Kenya for sending troops into Somalia against the extremists. Godane said at the time that the mall 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 kilometres south of Mogadishu, after hours of fighting.