A Somali military officer says heavily armed extremist fighters have launched an attack on an African Union military base in southern Somalia.

Col. Ahmed Hassan said the attack started Sunday morning when two suicide car bombs exploded at the entrance to the base and then armed fighters on foot tried to enter the heavily fortified base in Bulo-Marer, an agricultural town in Somalia's Lower Shabelle region.

He said the African Union troops were still fighting the militants around the base.

Somalia's Islamic extremist group, al-Shabab, claimed responsibility for the attack and said that its fighters killed dozens of African Union soldiers. African Union officials were not immediately available for comment.

Al-Shabab, which is fighting to impose Shariah law across Somalia, carries out suicide attacks across Somalia.