"There are a lot of casualties because the fighting started without any warning while people were preparing to break their Ramadan fast," he said.
The majority of the dead and wounded lived in a camp for displaced people in the south of the city, far from the districts that normally see fighting. Most were hit by mortar fire on the camp.
"Four of the dead, including three children, belonged to one family that had settled in the camp. One mortar bomb alone killed seven people," Duniya Mohamed, a witness to the attack, said.
Wracked by conflict
Al-Shabab, which has been accused of links to al-Qaeda, controls most of southern Somalia.
The group has been engaged in fierce fighting with the current 6000-strong AU force in Somalia, but the AU force has been restricted to retaliatory fire by the mandate it has been operating under.
Human rights groups warn that if the troops are allowed to expand their operations, civilians in Somalia will be further exposed to violence.
Somalia has been wracked by conflict for decades, with the latest bout of fighting erupting after Ethiopian troops, operating with US approval, invaded and overthrew the Union of Islamic Courts, whose rule had brought a period of relative stability to the troubled country.