Extremist insurgents battled for hours on Sunday with African Union troops after exploding a car bomb outside their base, Somali police, military and the militants said.

Al-Shabab militants attacked the African Union (AU) peacekeepers' base in the town of Bulomarer, 130 kilometers to the northwest of the capital Mogadishu, around 9 a.m. local time, residents from the area told Reuters.

Since withdrawing from Mogadishu in 2011, the al-Qaeda-linked group has lost control of most of Somalia's cities and towns. But it still retains a strong presence in regions outside the capital.

The militants initially detonated two car bombs that hit one AU vehicle and one Somali military vehicle, Somali army major Farah Osman, who is stationed near the AMISOM (African Union Mission in Somalia) base, said.

"Then a large number of al-Shabab fighters began firing from under the trees ... it was a hellish battle," he said, adding there was an unknown number of casualties.

The phone of the spokesman for the AMISOM force based in Mogadishu was switched off on Sunday and Reuters was unable to reach any other officials from the force for comment.

Abdiasis Abu Musab, a spokesman for al-Shabab said 14 of the group's fighters and 59 AMISOM troops were killed in the incident.

A police major stationed in a nearby town also said two car bombs exploded outside the base before the al-Shabab fighters entered it.

Major Nur Ali told Reuters that Somali and AMISOM forces had attacked al-Shabab in rural areas near the base on Saturday night. "Then al-Shabab attacked this morning as a revenge," he said.

Somalia has been mired in civil war since 1991.