Divisions emerged when al-Shabaab refused to recognize a power-sharing deal with Hizbul Islam.

Al-Shabaab militants have launched an offensive on the Hizbul Islam positions in the southern Somali port of Kismayo, where hundreds of families have fled their homes.

The pre-dawn attack triggered heavy fighting between the rival insurgent groups on Thursday in the relatively peaceful Kismayo, some 300 kilometers (185 miles) south of Mogadishu.

The once allied factions routed government forces in August to take control of the key port under a power-sharing deal.

But relations between the two groups soured in recent weeks after the rotating six-month rule they had agreed on failed upon al-Shabaab's refusal to relinquish the administration.

On Wednesday, many residents left the city center to seek refuge on the outskirts at the prospect of an imminent war between the rival groups who prepared for combat.

"We were attacked by our brothers with no reason," AFP quoted local Hizbul Islam spokesman Sheikh Ismail Haji Adow as saying.

Al-Shabaab fighters "launched their offensive on several fronts very early this morning. The fighting is very intense but we are holding up," he said.

Deepening division between al-Shabaab and Hizbul Islam could partly relieve President Sharif Sheikh Ahmed, whose western-backed administration has been seriously challenged by a massive military offensive from the country's main insurgent alliance since early May.

Al-Shabaab, once the military wing of the Union of Islamic Courts (UIC), has recently pledged allegiance to al-Qaeda's notorious leader Osama bin Laden.

Hizbul Islam is headed by Sheikh Hassan Dahir Aweys, a former ally of Sharif's in the UIC.

Somalia dipped in civil strife following the ouster of former dictator Mohammad Siad Bare in 1991, which led to a tug of power between rival factions.

presstv