Somali officials say an exchange of mortar fire has killed at least 13 people in the capital, Mogadishu.
At least 28 other people were wounded in the shelling Sunday between Islamist insurgents and pro-government forces.
Police officials say the insurgents fired several mortar shells at police headquarters in southern Mogadishu, killing one police officer and wounding three.
The officials say government soldiers and African Union forces returned fire, shelling Mogadishu's rebel-controlled area including the Bakara market.
At least 12 civilians were killed.
Islamist militants control large parts of the capital and southern Somalia after a nearly three-year insurgency.
They have vowed to overthrow Somalia's moderate Islamist government and replace it with a strict Islamic state.
Somalia's weak transitional government is under growing pressure to take tough measures against Islamist militants, especially al-Shabab.