Heavy fighting broke out Friday in the Somali capital of Mogadishu, as African Union-backed Somali soldiers reportedly made gains against al-Shabab holdouts in the city.

Somali officials said soldiers from the African Union Mission in Somalia (AMISOM) have taken over several of the insurgent group's strategic bases, in an attempt to drive the remaining militants from the city.

The AU peacekeeping force has been able to keep control of most of Mogadishu since al-Shabab abandoned its fixed positions in the city in August. But it has been unable to stop militants from carrying out a series of bombings and guerilla-style attacks in the war-torn capital.

Al-Shabab is fighting to overthrow Somalia's weak transitional government and impose strict Islamic law. It is battling AU forces in Mogadishu, as well as Kenyan soldiers in the south and Ethiopian troops in the west.

Mogadishu has seen near daily attacks by insurgents in recent days. On Thursday, a bomb exploded near a crowded area of makeshift shelters for refugees. The blast, which appeared to target a police checkpoint, killed six people.

The violence comes as Somalia on Friday marks six months since the United Nations first declared a famine in six regions of southern Somalia.

Although famine warnings have now been lifted in three of those regions, humanitarian groups say millions are still in urgent need of food aid.

But several humanitarian agencies have been forced to suspend aid to the region because of the violence.

On Thursday, Doctors Without Borders announced it was closing its two largest medical centers in Mogadishu after two of its workers were killed last month.

Last week, the International Committee of the Red Cross suspended aid to 1.1 million Somalis, saying that local authorities linked to al-Shabab had blocked aid to several regions.

The Islamist militant group has banned most foreign aid groups from operating in areas under its control.