Heavily-armed Al-Shabaab militants stormed Tuesday an African Union (AU) base in Somalia, triggering a fierce firefight that left an unknown number of casualties, a local military commander and witnesses reported.

AU forces dispatched helicopter gunships following a pre-dawn car bomb and gun attack on a camp housing Burundian peacekeepers near Ceel Baraf, a village about 160 kilometers (100 miles) northeast of the capital Mogadishu, the sources indicated. 

"There was heavy fighting and casualties inflicted on both sides, but we don't have more details about this incident so far," local military commander Mohamad Ali told AFP.

Ali added that Al-Shabaab militants "launched the attack with a car bomb blast before a heavy exchange of gunfire broke out."

Al-Shabaab has been waging a deadly insurgency against Somalia's central government for more than a decade.

The terrorist group issued a statement claiming responsibility for the assault and saying it had killed dozens of Burundian soldiers.

One witness said that "Al-Shabaab gunmen stormed the camp early morning, there were heavy blasts and exchanges of machinegun fire. The Burundians vacated the camp and entered Ceel Baraf village before helicopters arrived providing aerial support."

Long-delayed presidential election

African Union Transition Mission in Somalia (ATMIS) replaced the previous African Union Mission in Somalia (AMISOM) when its mandate expired at the end of March.

The new mission has the task of helping Somali forces take primary responsibility for security in the Horn of Africa nation.

Under a UN resolution, ATMIS is projected to gradually reduce staffing levels from nearly 20,000 soldiers, police, and civilians to zero by the end of 2024.

Before they were pushed out by the AU force, Al-Shabaab militants controlled Mogadishu until 2011. But they still hold territory in the countryside and frequently attack civilian, military, and government targets in Mogadishu and outside. 

The latest assault comes less than a week after Parliament chose new speakers for the upper and lower houses -- a key step on the path toward a long-delayed presidential vote.

Somalia's international partners have voiced fears that the delays were distracting the country from its myriad problems, including the fight against Al-Shabaab and the rising threat of famine in many parts of the country.