An exchange of heavy mortar fire between African Union forces and Islamist insurgents in Mogadishu left at least 12 civilians dead and scores wounded, officials and medical sources said on Monday.
The AU peacekeepers fired several mortar rounds into northern Mogadishu districts in retaliation for artillery attacks by the rebels late on Sunday.
"Our team collected eight bodies of civilians who were killed in the shelling and 55 others who were injured, some of them seriously," said Ali Musa, the head of the war-ravaged city's ambulance service.
Witness Abdulahi Nure said four other civilians were killed and their bodies ripped into pieces by the artillery fire in another neighbourhood.
"Around 20 rounds of heavy artillery shells hit Suqaholaha," said Mohamed Adan Ilbir, referring to his district.
"We are still shocked at this indiscriminate shelling," he added.
A police official who spoke on condition of anonymity told AFP: "They (rebels) fired mortar shells from the civilian populated areas using them as human shields."
At least 10 people were killed on Friday when the Al-Qaeda-inspired Shebab rebels fired a barrage of mortar shells at a ceremony to mark the first year of President Sharif Sheikh Ahmed's shaky rule.
The Shebab and Hezb al-Islam, another extremist militia, launched an offensive against the government last May, vowing to overthrow it as well as drive out the AU force, who they have also repeatedly attacked.
Sharif's government controls just a few streets in Mogadishu with the help of the AU peacekeeping missions, an understrength force yet to attain the intended capacity of 8,000 troops.
On Saturday, United Nations Secretary General Ban Ki-Moon ruled out any deployment of UN peacekeepers in Somalia until the end of the country's years of civil war.
"Practically and realistically it is not possible at this time to deploy a UN peacekeeping force in Somalia," he told reporters in Addis Ababa. "We need a peace to keep and now there is no peace."
The AU has called several times on the UN to take over from its own beleaguered peacekeeping force which has been powerless to stop fighting between Islamist rebels and a weak transitional government.