At least four people were killed in Somalia's capital Mogadishu when a suicide bomber rammed a car packed with explosives into a UN convoy.

The convoy of armoured vehicles were reportedly 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.

The attack was not immediately claimed, but both the airport and the UN mission have been frequently targeted in the past by Somalia's al-Qaeda-affiliated al-Shabaab rebels, who are fighting to overthrow the country's internationally-backed government.

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

Witnesses said the UN convoy consisted of four armoured vehicles escorted by private security personnel driving in pick-ups.The attack appeared to be a repeat of an al-Shabaab operation in February, when six people 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 al-Shabaab last year.

The airport zone is also the base of the 22,000-strong African Union force fighting al-Shabaab militants.

Al-Shabaab 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.

Al-Shabaab leader Ahmed Abdi Godane was killed by a US air strike in September.

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 occurrence in the city.

Al-Shabaab has also stepped up operations in Kenya, and yesterday massacred 36 non-Muslim quarry workers in a Kenyan border town.