The attack took place 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.
"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 said.
"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.
"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.
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"It is difficult to go nearby as police are blocking the road," Liban told AFP.
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 Shebab.