Fresh clashes between one-time Somali rebel allies, al-Shabaab and Hizbul Islam, over a strategic port town has left at least 25 people dead.

The fighting erupted less then 48 hours after the warring sides signed a fragile ceasefire.

In the past two weeks, fighters from both groups have clashed over strongholds in Somalia's southern port of Kismayo, 500 km from the capital Mogadishu, which was formerly shared by the two.

Last week, al-Shabaab finally seized control of the town and appointed its own government there.

However on Saturday, Hizbul Islam militants launched an offensive on the al-Shabaab positions in a pre-dawn attack, triggering intense fighting that left at least 25 people dead and more then 41 others injured, Press TV correspondent reported on Saturday.

The fresh spate of attacks has forced hundreds of families to flee their homes, worsening the humanitarian crisis in the region.

Relations between the two groups -- which had joined forces against the new UN-backed government in Mogadishu and had managed to take control of Kismayo -- soured after the rotating six-month rule they had agreed on failed upon al-Shabaab's refusal to relinquish administration.

Meanwhile, after a brief lull in the fighting with the UN-backed government, massive explosions and heavy artillery fire have once again rattled the war-wracked capital, claiming an unknown number of lives.

According to witnesses, the attacks mainly targeted African Union (AU) and Somali government bases in Mogadishu.

Though no group has yet claimed responsibility; police warned on Thursday that al-Shabaab was planning more attacks on AU troops.

The 5,100-strong AU Mission in Somalia (AMISOM), which began its operations in March 2007, is made up of soldiers from Burundi and Uganda and is mandated to guard strategic sites in the volatile Somali capital.

The mission also provides backup to government forces fighting the violent insurgency.

Mogadishu has witnessed fierce fighting and attacks on a daily basis since May, when rebel forces launched a major offensive against President Sheikh Sharif Ahmed's interim government.

The mission is the only foreign force currently present in Somalia, a country which has been mired in civil war since 1991.