The ambush took place on Sunday outside the southern town of Baidoa - an area which has seen fierce clashes between two rival militias.

One newspaper reported that the death toll was as high as 28.

Fighting could jeopardise food aid

It said the ambush - which left another 10 injured, some seriously - was carried out by the Rahanwein Resistance Army (RRA).

Bidoa has long been a focus of fighting between the RRA and warlord Hussein Mohamed Aidid's forces, which control Bidoa.

The ambush was carried out on the road to the capital Mogadishu. Newspapers say the Rahanwein militia had recently closed the route to passenger vehicles and humanitarian operations.

The attack is one of the worst incidents in the area for some time. Reports say the passengers on the bus were civilians from subclans loyal to both sides.

'Bus was carrying arms'

However, RRA spokesman Mohamed Aden Qalinle, who confirmed the attack, told the French news agency AFP that the bus had regularly violated RRA warnings forbidding vehicles from using the road.

He denied the bus was only transporting civilians and accused gunmen inside the vehicle of starting a shootout.

Mr Aidid condemned ''cowardly'' attack

He said the bus was transporting small arms and other logistical supplies for Mr Aidid's militiamen.

"Six gunmen with heavy machine guns were at the top of the bus and opened fire at an RRA checkpoint after refusing to stop," he was quoted saying.

But Mr Aidid's supporters described the killing as an "inhuman and cowardly act".

The attack comes as relief agencies warn that the continuing fighting could prevent food reaching thousands of people in need.

The World Food Programme delivered food aid to some 6,000 people in the Bidoa area in the middle of December and is expected to deliver to thousands more.

But it is concerned that the instability could cause problems for its road convoys.