About 8 000 African Union troops protecting Somalia's government from Islamist rebels are now deployed in Mogadishu, scene of sustained fighting this week, a spokesman for the force said on Thursday.

The latest reinforcements take the peacekeeping force, known as AMISOM, to the upper limit of its United Nation's-authorised mandate, although African nations have said a 20 000-strong force is needed to rout the militants from the capital.

"We have four battalions from Burundi and five battalions from Uganda, totalling slightly over 8 000. We still need troops to enable us to eject them (militants) from the places in Mogadishu from where they launch their attacks," AMISOM's spokesman in Mogadishu, Barigye Ba-hoku, told Reuters.

UN Security Council members supported on Tuesday the idea of increasing the AU's mission in the lawless Horn of Africa country, an indication it may approve a significantly bolstered force.

Uganda's President Yoweri Museveni, who last weekend became the first serving head of state to visit Somalia since 1991, says his nation could provide all the troops needed. But he wants the international community to foot the bill as he prepares for Ugandan presidential elections in February.

Ambulance workers said that in the last three days at least 14 civilians had been killed, and dozens wounded, in the cross-fire between AU-backed government troops and the militants fighting to topple the interim government.

"Most of the casualties took place in the north of Mogadishu where the warring groups have been blindly exchanging shells," said Ali Musa, coordinator of ambulance services.

Separately, the two rebel groups fighting the government together in Mogadishu – al Qaeda-linked al Shabaab and Hizbul Islam – clashed between themselves for a second day over control of Burhakaba, a town in the south of the country.

"Hizbul Islam attacked this morning with many battle wagons," said resident Halima Suleiman, referring to pick-up trucks armed with heavy machine guns. "After fierce fighting, al Shabaab were driven out of the town."

Some locals said at least eight people had been killed.

At least 21 000 civilians have been killed since the start of the insurgency. Aid agencies and rights groups have become increasingly concerned about indiscriminate shelling and some have accused combatants on all sides of war crimes.

The United Nations has appealed for $530-million next year to tackle Somalia's humanitarian crisis, one of the world's worst, which has left about a quarter of the population dependent on aid.