Shebab militants claimed to have overrun an African Union army base in a village in southern Somalia on Friday, in the latest attack during the insurgents' annual Ramadan fighting season.

The military outpost in Lego village, 100 kilometres (62 miles) northwest of the capital Mogadishu, is manned by Burundian soldiers who are part of the 22,000 strong African Union Mission in Somalia (AMISOM).

Residents said the dawn raid started with a suicide bomber driving a car loaded with explosives into the entrance of the base, before dozens of militants armed with machine guns and rocket propelled grenades began their assault.

"We could see smoke and hear explosions, there was heavy fighting in the AMISOM base with heavily armed Shebab fighters," said Hassan Mohamed, a resident.

Several other witnesses confirmed heavy fighting at the base.

There was no immediate confirmation of casualties, but a Shebab spokesman claimed the militants had taken control of the base killing "dozens", and had raised their flag and seized weapons.

"The Black Islamic flag is flying over the main base of AMISOM in Lego this morning and the dead bodies of the enemy are scattered around the area, the mujahedeen fighters have taken all their military supplies," Mohamed Abu-Yahya, a Shebab commander said.

In a statement AMISOM confirmed "an ongoing attack" at its Lego base.

The Al-Qaeda affiliate is fighting to overthrow Somalia's Western-backed government which is propped up and protected by AMISOM.

Under military pressure at home Shebab is increasingly turning its attention to Kenya where it has launched a string of recent deadly assaults.

The Islamic militants commonly step up their attacks during Islam's holy fasting month of Ramadan.