At least four people were killed in Somalia’s capital Mogadishu on Wednesday when a suicide bomber rammed a car packed with explosives into a UN convoy, police said.

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

The convoy of armored vehicles was 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 Shebab militants, who are fighting to overthrow the country's internationally-backed government.

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

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

"I saw four dead people so far, but the casualties are believed to be more, we are still investigating the incident and there is chaos in the area," Liban added.

A dark plume of smoke was seen rising high in the sky after the large blast, which was heard across the seaside capital.

Witnesses said the UN convoy consisted of four armored vehicles escorted by private security personnel driving in pickups.

"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, but it is difficult to go nearby as police are blocking the road," said Shamso Idle, a local resident.