At least 13 civilians died and more than 35 were wounded during the latest round of heavy fighting between Islamic insurgents and African Union (AU) peacekeepers in the Somali capital Mogadishu, witnesses and officials said Wednesday.

Islamist forces attacked the Ugandan KM4 base in Mogadishu late on Tuesday night, prompting AU troops to respond with heavy artillery and tanks.

Some of the AU fire landed in residential neighbourhoods, killing civilians, witnesses said.

"Five people died when a shell landed in building where people were sheltering," Hassan Osob Omar, a resident in the Bakara market area, told Deutsche Presse-Agentur dpa.

Another shell landed at a former university housing thousands of internally displaced people.

Three died in the building, including a mother of five, and five others were wounded, Suado Hashi Roble, a resident, told dpa.

Four people from another family died when a shell landed at Taleeh village.

Dahir Mohamed, deputy director of Madina Hospital told dpa that 35 people had been admitted to hospital on Wednesday morning, but he expected this figure to rise.

No AU casualties were reported.

Heavy clashes have been ongoing since insurgents shelled Mogadishu airport last Friday as an AU plane defied a flight ban.

No planes had landed since Tuesday after insurgent group al- Shabaab said it would destroy any aircraft that attempted to touch down at the airport.

Clashes around the airport and in the heavily populated Bakara market, which is seen as a stronghold of Islamist insurgents, have since claimed the lives of over 30 people.

Thousands are reported to be filing out of Mogadishu to escape the battles - the worst sustained violence for months.

Near the KM4 camp on Wednesday morning, hundreds of people could be seen fleeing on foot with their possessions stacked into wheelbarrows.

Hundreds of thousands have already fled and are living in makeshift camps outside Mogadishu or in the Dadaab refugee complex in the east of neighbouring Kenya.

The Mogadishu-based Elman Peace and Human Rights Organization this week said that a total of 9,474 civilians have died in the insurgency since early 2007.

Almost daily battles have blighted the Horn of Africa nation since Ethiopian troops invaded in 2006 to kick out the Islamist regime and put the transitional federal government back in power.

The government and moderate opposition figures have signed a peace agreement, with the technical details of the ceasefire still being hammered out, but al-Shabaab has rejected the deal.

Ethiopian troops must leave before any peace can be negotiated, al-Shabaab says.

The Horn of Africa nation has been plagued by chaos and clan-based civil war since dictator Mohamed Siad Barre was toppled in 1991.