A suspected suicide car bomb has killed at least 22 civilians in the Somali capital Mogadishu, witnesses say.

The attack on Saturday was apparently aimed at a group of African Union peacekeepers but missed its target, they said. 

"That opposition group has massacred only innocent Somali people," Major Barigye Ba-hoku, spokesman for the AU force, said.

The attack came just days before more troops from Uganda and Burundi were due to arrive to boost the 1,400 African Union peacekeepers currently deployed in Somalia.

Abdifatah Shaweye, the city's deputy governor, told the Reuters news agency that police officers stationed near the base had opened fire on the car as it approached, after which it crashed and blew up.

Gunfights were reported to have broken out after the car exploded.

Mohamed Osman Ali, Mogadishu's mayor, said it was unclear who was behind the attack.

Doctors said at least 30 other people were wounded.

Abdifatah Ibrahim Shaweye, Mogadishu's deputy governor told the AFP news agency that the bomber was a foreigner.

"We have one of his arms which is clearly showing that the suicide bomber was a foreigner" Abdifatah Ibrahim Shaweye, Mogadishu's deputy governor

"We have one of his arms which is clearly showing that the suicide bomber was a foreigner," he said, explaining that the bomber's light skin tone showed he was not Somali.

Somalia is wracked by violence with near-daily attacks on troops loyal to the largely powerless UN-backed transitional government.

Much of the country is controlled by armed opposition groups who have captured many of the towns and villages seized by government and Ethiopian troops from the Islamic Courts Union in late 2006.

The interim government has failed to bring stability to the Horn of Africa nation, where more than 16,000 people have been killed in the past two years and one million others driven from their homes.

Some analysts have said the the recent withdrawal of Ethiopian troops could create a power vacuum as opposition forces scramble for control.