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

"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 Mohamed Ali told AFP by phone.

"They launched the attack with a car bomb blast before a heavy exchange of gunfire broke out," he added.

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

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

Its claim could not be verified and there was no immediate comment from the AU's transition mission in Somalia, or ATMIS.

"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," said one witness, local resident Weliyow Maalim.

"The helicopters fired missiles and heavy machineguns, we saw smoke rising over the camp but we don't know about the situation exactly," said another witness, Ahmed Adan.

- New AU mission -

ATMIS replaced the previous AMISOM peacekeeping force 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 troubled 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.

Al-Shabaab fighters controlled Mogadishu until 2011 when they were pushed out by the AU force.

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 towards a long-delayed presidential vote.

The drawn-out election process has been marred by deadly violence and a bitter power struggle between the president and prime minister.

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