"Dozens" of Burundian soldiers were killed in an attack on an African Union peacekeeping base in Somalia, according to a security source in the region and a Mogadishu-based source, after the deadliest attack on the peacekeepers in five years.

The security source in the region said several follow-up operations were going on after Tuesday's attack by al Qaeda-linked al Shabaab militants. The total number of casualties was not yet clear, he said, but "dozens" of soldiers were killed and about 20 were evacuated with wounds.

The African Union (AU) typically does not release casualty numbers after an attack. Neither do troop-contributing countries. Neither Burundi nor the AU responded to requests for comment.

The source verified the authenticity of a video showing five soldiers lying dead - most had been stripped of their boots - as Somalis wandered inside the base perimeter. One soldier appeared to have been beheaded.

There had been between 150-200 Burundian soldiers on the base, he said, but no Somalis because Monday was the Muslim Eid holiday.

More than 100 soldiers had been accounted for, he said.