"We collected two dead bodies and four injured yesterday in and around Bakara market," Ali Muse, an ambulance service coordinator, told Reuters.

Residents in Mogadishu's Towfiq area said they saw the bodies of three suspected insurgents being pulled from a house they had lived in. "Three dead men's bodies were removed from a house in our village," Muse, a resident in Towfiq, told Reuters. "A mortar shell struck their house and smoke was rising over it when they were evacuated."

Mohamed Abdullahi, a resident of north Mogadishu, said three people were injured after a mortar landed where they were taking cover from the heavy shelling on Monday night.

Western security experts say the failed Horn of Africa state has become a safe haven for hardliners, including foreign jihadists, who are using it to plot attacks across the region and beyond.

Human Rights Watch (HRW) said on Monday that western powers and the United Nations have been quick to condemn civilian killings by Somali Islamist fighters but have turned a blind eye to abuses by government troops and African Union soldiers.

"The U.S. government has sent mortars to transitional government forces in Mogadishu even though no party to the fighting has used the weapons in accordance with the laws of war," the rights group said in a statement.

Kenya was guilty of recruiting Somali youths from refugee camps to be fighters, contravening the humanitarian status of the camps, the group said.

HRW also accused the rebels of brutally repressing Somalis in the vast regions of south and central Somalia that they control.

The group said that while al Shabaab had brought a degree of stability to areas they controlled, they routinely killed civilians and meted out harsh punishments including amputation.

It cited cases of women being flogged or jailed for selling tea to support their families because the work brought them into contact with men.