A US air strike in Somalia killed at least six members of the Islamic extremist group al-Shabaab, possibly including its leader who was in a car that was hit.

Ahmed Abdi Godane has no heir apparent as leader and analysts fear his death will spark further instability in the group.

The al-Shabaab leader was in one of two vehicles hit on Monday night by the strike, a commander of the Somali Islamic extremist group said, but he would not say whether Godane was among the six killed.

The vehicles were heading towards the coastal town of Barawe, al-Shabaab's main base, when they were struck, Abu Mohammed said. The Pentagon confirmed that US military forces had attacked the extremist network in Somalia on Monday.

The strike caused ground-shaking explosions, a witness said. Somali government and African Union forces heading to a town in the area heard what sounded like an earthquake as al-Shabaab's bases were hit, the governor of Somalia's Lower Shabelle region, Abdiqadir Mohamed Nor, said. "There was an airstrike near Sablale. We saw something," he said.

A year ago this month al-Shabaab attacked the Westgate shopping centre in Nairobi, Kenya, killing at least 67 people, and the US targeted the planners of the bloody assault.

The US drone strike was launched as Godane left a meeting of the group's leadership, said a senior Somali intelligence official. Intelligence indicated Godane "might have been killed along with other militants", said the official, speaking on condition of anonymity.

The attack took place 105 miles (170km) south of Mogadishu, at a site where al-Shabaab trains its fighters, he said.

Godane, also known as Mukhtar Abu Zubeyr, is al-Shabaab's spiritual leader under whose direction the Somali militants forged an alliance with al-Qaida. In 2012 the US offered a reward of up to $7m (£4.2m) for information leading to his arrest.

US commanders said they are waiting to determine the outcome of the air strike. "We are assessing the results of the operation and will provide additional information as and when appropriate," said a Pentagon spokesman.

After the US strike in a forest near Sablale district masked Islamist militants in the area arrested dozens of residents they suspected of spying for the US and searched nearby homes, a resident said.

"Mass arrests just started, everyone is being detained," said local resident Mohamed Ali. "They even searched nearby jungles and stopped the nomads transporting milk and grass to the towns for questioning."

The US has carried out several air strikes in Somalia in recent years. A US missile strike in January killed a high-ranking intelligence officer for al-Shabaab and last October a vehicle carrying senior members of the group was hit in a strike that killed al-Shabaab's top explosives expert.

The latest US action comes after Somali government forces regained control of a high-security prison in the capital that was attacked on Sunday. Seven heavily armed suspected al-Shabaab 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 bomber detonated an explosives-laden vehicle at the gate of the prison and gunmen then fought their way inside.

Al-Shabaab attacked the shopping centre in Nairobi last year to punish Kenya for sending troops into Somalia against them. Godane said at the time that the mall attack was carried out in retaliation for the west's support for Kenya's intervention in Somalia and in the "interest of their oil companies".

The group is mostly active in Somalia's rural regions after being ousted from the capital by African Union forces in 2011.

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