UNHCR said 600,000 people were believed to have fled Somalia's lawless capital Mogadishu since February, when clashes pitting allied Somali-Ethiopian troops against suspected Islamist insurgents started to escalate.

Nearly 200,000 people have streamed out of Mogadishu in the past two weeks alone, emptying entire neighbourhoods, UNHCR said in a statement.

The numbers of displaced persons this year are in addition to 400,000 forced to flee their homes because of previous fighting.

The Horn of Africa country has been in a state of anarchy since warlords toppled military dictator Mohamed Siad Barre in 1991.

The latest conflict flared when Islamist leaders took control of various towns in southern Somalia last year before being ousted in January by government soldiers and their Ethiopian allies.

The interim government has since faced an Islamist insurgency, featuring roadside bombings and political killings, that has all but thwarted its attempts to restore central rule.

Civilians typically bear the brunt of the violence.

UNHCR said the number of internally displaced persons (IDPs) living in more than 60 makeshift settlements along the 30 km (20 miles) stretch of road linking Mogadishu to Afgoye had soared to nearly 200,000 -- a 50 percent increase in the past two weeks.

"Families continue to lack proper shelter and consistently resort to using any material -- mostly plastic bags and rags -- to patch up their 'tukuls' -- flimsy dome-shaped shelters," UNHCR said.

"Although IDPs express confidence in security in the Afgoye area, we are increasingly worried about security incidents there in the last several days," it said.

UNHCR said an aid worker was killed on Friday after she was hit by a stray bullet while distributing supplies. On Sunday, an explosion in Afgoye town killed six people, it added.