At least 20 people were killed and more than 30 others injured on Friday in fresh fierce fighting that rocked parts of Somalia 's restive capital Mogadishu, medics and witnesses said.

Witnesses said clashes erupted on the evening in Mogadishu 's northern districts of Karan, Shibbis and Hodan neighborhood where several mortar shells exchanged by the warring forces landed in residential areas.

Ali Muse of Mogadishu Ambulance Service says the death toll may rise as most of the wounded are in critical condition. He added that those killed are most of them civilians.

Most of the wounded civilians were admitted in Mogadishu 's Keysaney hospital, according to ambulance workers.

Al-Shabaab insurgent group claimed responsibility for the fighting, saying it carried out attacks in government forces in the districts.

The violence breaks several days of lull in the chaotic capital of the war-torn country, where tension still remains high over a government-planned onslaught against the rebels.

But the offensives would be done gradually because of lack of capacity, according to Somali officials.

Somalia has been without an effective central government since 1991 and, almost half of its population-- 3.8 million people--are in dire need of humanitarian food aid.