MOGADISHU - Four people were killed and nine wounded in Somalia's capital Mogadishu on Wednesday when a suicide bomber rammed a car packed with explosives into a UN convoy, police and rebels said. The convoy of armoured vehicles were ferrying staff between Mogadishu's heavily-fortified airport and a protected UN base in the city when it was hit close to the airport gate.

Somalia's Al-Qaeda-affiliated Shebab rebels, who are fighting to overthrow the country's internationally-backed government, said in a statement that their fighters had "targeted a convoy of foreign mercenaries and their apostate allies."

UN convoys are systematically flanked by pickup trucks carrying private security officers who provide protection for international staff.

"The bomber drove in between the security escort and the UN armoured vehicles and detonated the car, ramming into one of the escort vehicles," police officer Mohamed Liban told AFP.

Liban said he had counted four bodies, all Somali nationals either providing security for the UN or those passing by who had been caught by the blast. Police said nine others were wounded. The UN mission in Somalia confirmed the attack, saying in a statement that there were "thankfully no injuries to UN staff" but regretting the "casualties amongst bystanders and security personnel." A dark plume of smoke rose high in the sky as fierce flames engulfed the wreckage of vehicles hit by the explosion, which was heard across the seaside capital. Witnesses said the UN convoy consisted of four armoured vehicles escorted by private security personnel driving in pick-up trucks. "The explosion was very big and there is smoke all around the area. I can hardly see people lying on the ground, either dead or wounded," said local resident Shamso Idle.

The attack appeared to be a repeat of a Shebab operation in February, when six people - Somali guards, passers-by and shop owners - were killed in a suicide attack on a convoy carrying UN staff near the airport.

A number of foreign diplomatic missions are based inside the huge airport complex, which has also been used to house a number of UN staff since a city-centre UN compound was attacked by the Shebab last year.

The airport zone is also the base of the 22,000-strong African Union force fighting the Shebab. Shebab fighters once controlled most of southern and central Somalia, but have been driven out of fixed positions in Mogadishu and most major towns by the AU force.

Shebab leader Ahmed Abdi Godane was killed by a US air strike in September. He has since been replaced by Ahmad Umar Abu Ubaidah.

The group have carried out a string of high profile attacks in Mogadishu this year, including against the presidency, parliament and intelligence headquarters.

Shootings and car bombings are also a regular occurence in the city.

The Shebab have also stepped up operations in Kenya, and on Tuesday massacred 36 non-Muslim quarry workers in a Kenyan border town.

Kenyan President Uhuru Kenyatta vowed to "intensify the war on terrorism" against the Shebab following the quarry attack, calling the Islamists "deranged animals" and blaming them for the death of more than 800 Kenyans. The Shebab in turn warned they would be "uncompromising, relentless and ruthless" in further attacks.