New battles have broken out in the Somali capital, Mogadishu, killing at least 13 people.
Gunmen have controlled Mogadishu for 15 years
Militia of the Islamic Courts attacked and captured a garage where their rivals loyal to a group of secular warlords were based in the north-east.
Earlier in the week the warlords calling themselves the Anti-Terrorism Alliance seized a hospital in the area, forcing dozens of wounded to flee.
More than 200 people, mainly civilians, have died in the recent fighting.
Somalia has had no effective government since the overthrow of President Siad Barre in 1991 but this year's clashes have been the worst in the capital for more than a decade.
A truce agreement between the two rival militias was broken last week.
"We woke up to the sounds of the gunfire and after an hour the whole area fell into the hands of the Islamists including a key check point on the road that links the city to the central regions," Jamaal Ali, the owner of the garage where the warlords had set up their base, told the BBC.
A local butcher at the battle zone, which was near a livestock market, said some 300 Islamist militias with 30 technical cars were involved in the surprise attack.
He told the BBC that after morning prayers, some of the Islamists hid themselves inside a truck covered with plastic sheeting and managed to pass through the first defensive positions of the alliance fighters.
Five people died in hospitals run by the International Committee of the Red Cross and eyewitnesses say eight dead bodies were lying in the streets.
Nineteen wounded were admitted to the nearby Shiffo Hospital.
The BBC's Mohammad Olad Hassan in Mogadishu says hundreds of people have began to flee from their homes fearing a counter-attack
Other people in the area demonstrated in the streets against the fighting - some in support of the Islamic Courts, saying that the militia loyal to the warlord Botaan Iisa Aalin had been harassing them.
Our correspondent says this is the third base that the militias loyal to the Islamic Courts have captured from warlords.
Mogadishu's medical facilities are struggling to cope
This latest incident means that the Islamists now control a key check-point linking Mogadishu to the central regions of Somalia.
Over the last few weeks, the Islamic Courts' militia have gradually been gaining the upper hand in the fighting, which has forced many civilians to hide in their homes or seek shelter elsewhere.
The UN's humanitarian co-ordinator for Somalia has warned the militias that their actions may be considered war crimes under international law.
The violence began earlier this year when a group of warlords, who had divided Mogadishu into fiefdoms, united to form the Anti-Terrorism Alliance to tackle a growing Islamist force.
The Anti-Terrorism Alliance includes eight warlords, four of whom are ministers in the current government.
A senior US diplomat specialising in Somalia, Michael Zorick, has been removed from his post in Kenya after expressing concerns about US support for the Mogadishu warlords, who say the Islamic Courts are sheltering al-Qaeda fighters.
The US merely says it will support those trying to stop "terrorists" setting up in Somalia but stresses its commitment to the country's transitional government, which functions from Baidoa, 250km (155 miles) north-west of the capital.
Are you in Mogadishu, or do you know anyone who is? Please use the form below to send us your experiences and contact details.
The BBC may edit your comments and not all emails will be published. Your comments may be published on any BBC media worldwide.