Nairobi/Mogadishu - The death toll in a double suicide bombing on a peacekeeping base in Somalia's capital city has risen to 21, including 17 peacekeepers, a spokesman for the African Union mission in Somalia AMISOM said Friday.

Two white cars bearing United Nations logos, but driven by Islamist insurgents battling the Western-backed government, gained access to the AU base at Mogadishu's main airport on Thursday, detonating once they were inside.

"We have 21 dead so far: 17 peacekeepers, - 12 Burundians and five Ugandans - and four civilians," Gaffel Nkolokosa, a Nairobi-based spokesman for AMISOM, told the German Press Agency dpa. "There are also about 40 injured."

The deputy force commander, Major General Juvenal Niyonguruza from Burundi, was amongst the dead.

Heavy fighting broke out between insurgents and government forces following the bombings, with mortars raining down on residential areas in Mogadishu. Over a dozen people were killed in the exchanges, local media reported.

The 5,000-strong peacekeeping force, dubbed AMISOM (African Union Mission to Somalia), is made up of troops from Burundi and Uganda.

Insurgent group al-Shabaab claimed responsibility for the attacks, saying it was targeting government and AU officials who it said were planning future assaults against Islamist forces.

Al-Shabaab, which is fighting the Western-backed government, on Tuesday vowed to retaliate after a US raid on Somali soil killed an al-Qaeda suspect and several al-Shabaab fighters.

Kenyan-born Saleh Ali Saleh Nabhan, wanted in connection with several terrorist attacks in neighbouring Kenya, was killed by US forces in a strike on Somali soil on Monday.

The US has been tracking Nabhan since the 2002 bombing of an Israeli-owned hotel in the Kenyan coastal town of Mombasa, which claimed 15 lives.

He was also suspected of being behind an almost-simultaneous failed attempt to shoot down an Israeli airliner as it left Mombasa.

The head of AMISOM, Nicolas Bwakira, condemned the "barbaric attack", but said that the AU was still committed to supporting Somalia's transitional government.

The peacekeepers are propping up the government in Somalia, which has been embroiled in chaos since the 1991 ouster of dictator Mohamed Siad Barre.

Together with its ally Hizbul Islam, al-Shabaab - which the US says has close links with al-Qaeda - has been battling to remove Western-backed President Sheikh Sharif Sheikh Ahmed.

The current insurgency kicked off in early 2007 following an Ethiopian invasion and has recently gathered pace.

The attack on the AMISOM base is one of al-Shabaab's most ambitious attacks in months, although it is not the first time the militants have claimed the lives of peacekeepers. Eleven Burundian peacekeepers died in an attack in February.

More than 250,000 people have fled renewed fighting in Mogadishu since May, bringing the total number of displaced within Somalia to over 1.5 million.

More than 18,000 people have died since the insurgency began. Over half of the Somali population are now dependent on food aid due to the conflict and drought. (dpa)