Mogadishu - A suspected car bomb exploded close to the heavily fortified gates of the airport in Mogadishu, the entrance to the "green zone" and base of foreign embassies in the Somali capital, witnesses said on Wednesday.
A dark plume of smoke was seen rising high in the sky after the large blast, witnesses told AFP.
Police said at least four people were killed.
"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," police officer Mohamed Liban said.
"The bomber drove in between the security escort and the UN armoured vehicles and detonated the car, ramming into one of the escort vehicles," Liban said, adding he had seen four dead bodies but feared more may have been killed.
A dark plume of smoke was seen rising high in the sky after the large blast, which was heard across the seaside capital.
"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 witness Shamso Idle.
"It is difficult to go nearby as police are blocking the road," he added.
There was no immediate claim of responsibility, although al-Qaeda-affiliated Shebab rebels have carried out a string of bombings and attacks in the city.
Four UN armoured vehicles were reportedly ferrying staff either from or to the airport to a protected UN base in the city. The airport zone is also the base of the 22 000-strong African Union force, which is fighting the Shabaab.
The UN convoy was flanked by pickup trucks carrying private security officers who provide protection for international staff in the dangerous city.