MOGADISHU: Fierce battles between government forces and Islamist rebels in Mogadishu forced some 8,000 people to flee the Somali capital in one day, the United Nations said Sunday.

"Unfortunately, due to heavy fighting on Friday 22nd, the number of displaced from Mogadishu sharply increased," the office of the UN High Commissioner for Refugees said.

"An estimated 8,000 were displaced on Friday only, bringing the total number to 57,000," the UNHCR said in a statement.

At least 31 people were killed on Friday, most of them civilians trapped in the crossfire or claimed by mortars in one of the deadliest days since a new round of fighting broke out more than a fortnight ago.

President Sharif Sheikh Ahmed is holed up in his compound with a handful of supporters and under African Union peacekeepers' protection, while his forces have sought to drive out rebels from their strongholds in the city.

The Shebab and Hezb al-Islamiya fighters are the main insurgent groups trying to topple Sharif's internationally recognised transitional government.

Some 150 people have been killed overall since the rebels launched a fresh round of attacks on May 7 saying they had received the support of foreign fighters.

At least civilians were killed in clashes on Saturday but an AFP correspondent said there had been no fighting on Sunday by midday.

The Somali capital has been ravaged by 18 years of almost uninterrupted civil conflict and emptied of hundreds of thousands of residents by the violent fighting that followed Ethiopia's 2006 invasion.

Somalia has not had a central government since the ouster of President Mohamed Siad Barre in 1991 which set off an interminable and bloody cycle of clashes between rival factions.