Al-Shabab is fighting a relatively new rival Islamist militant group
At least 40 people have died in clashes between Islamist groups vying to control key towns in central Somalia.
Fifty others were wounded as a self-styled moderate Islamist group, Ahlu Sunna Waljamaca, continued to battle militants from the hardline al-Shabab.
In the capital, Mogadishu, 10 people died as insurgents fired mortars at the bases of government troops.
Monday's violence came as Somalia's president quit, adding to the chaos as Ethiopian troops prepare to withdraw.
Ahlu Sunna Waljamaca, a relatively new Islamist group, has declared a holy war on al-Shabab, blaming it for the bloodshed in Somalia.
It seized two towns in central Somalia over the weekend, triggering fierce clashes.
Various Islamist and nationalist groups control most of southern Somalia. Government forces only control parts of the capital, Mogadishu, and the town of Baidoa.
The bloodletting continued as Somalia's President Abdullahi Yusuf quit following a power struggle with the prime minister.
Somalia's president quit after a power struggle with the premier
The pair had clashed in recent months over attempts to negotiate a peace deal with moderates in the Islamist-led armed opposition.
In 2006, President Yusuf made the unpopular decision to call in troops from neighbouring Ethiopia to prop up his fragile administration but the move has failed to quell the Islamist insurgency.
Ethiopian troops are due to pull out this week, raising fears of a power vacuum in a country that has not had an effective national government since 1991.
Fighting between the Ethiopia-backed government and the insurgents has left one million people homeless and much of Mogadishu deserted.