If the two groups fall out nationally, Somalia's woes could worsen
Fierce clashes have broken out between two Somali militant Islamist groups near the southern port of Kismayo, breaking a recent ceasefire deal.
An al-Shabab spokesman said his group had fought off an attack from the Hizbul-Islam group, killing some of their fighters.
There has been no independent verification of his claims.
The two groups began fighting over the port at the start of this month, but agreed a truce shortly after.
The militants had ousted government troops from Kismayo last year and agreed to govern the port together.
But al-Shabab named an administration last month which excluded Hizbul-Islam, sparking the fighting.
After a truce for most of this month, al-Shabab spokesman Sheikh Hassan Ya'qup said Hizbul-Islam attacked his fighters in Birta, 60km (37 miles) north of Kismayo.
"We repulsed them - they left some dead bodies behind," he said.
Analysts said it was the first time Hizbul-Islam had been on the offensive in the dispute.
Their members were reported to have fled Kismayo after at least 12 people were killed in fighting at the start of the month.
The country has been wracked by conflict since 1991, when it last had an effective national government.
Islamist militants and warlords control much of southern Somalia, fighting against government troops.
The government, backed by the UN and African Union, controls only small parts of the capital, Mogadishu.
Some three million people - half the population - need food aid, while hundreds of thousands of people have fled the country.