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Most of those who died were passers-by and several UN body guards were wounded in the explosion, the witnesses said.
The al-Shabab Islamist group has said it was behind the explosion in one of the most heavily fortified areas in the city - where many embassies are based.
The blast was near a checkpoint at the entrance to the airport complex, the AFP news agency reports.
"At least six people, most of them civilians, died in the car bomb explosion. There are many casualties, serious injuries," police officer Said Mohamed told AFP.
Eyewitnesses say the blast did only superficial damage to the UN vehicles.
Correspondents say there has been a spate of attacks in Mogadishu in the last week, with gunmen targeting Somali army bases in the city, and several artillery shells, fired from about 6km (nearly four miles) away, have landed near the airport.
Residents also report an increase in sustained night-time attacks, with mortar and gunfire, on the northern outskirts of the city.
The airport is the main base for the African Union force which is based in Somalia to help the fledgling UN-backed government gain control of the country from al-Shabab.
The insurgent group has lost control of most main cities in the last two and half years, but it still holds sway in smaller towns and rural areas in southern Somalia.
The airport complex is also where the African Union force is based
At least six people have
died after a car bomb exploded close to the gate of the international
airport in Somalia's capital, Mogadishu, police say.
Eyewitnesses told the BBC it was targeting a UN refugee agency convoy.Most of those who died were passers-by and several UN body guards were wounded in the explosion, the witnesses said.
The al-Shabab Islamist group has said it was behind the explosion in one of the most heavily fortified areas in the city - where many embassies are based.
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This apparent increase in militant activity in and around
Mogadishu comes amid warnings that weapons destined for the Somali
security forces are instead making their way into the hands of clan
militia and even al-Shabab fighters.
Last year the United Nations partially lifted an arms embargo, in place since 1992. The move was supported by the US and the UK and was aimed at bolstering the Somali police and army with a limited supply of relatively small weapons such as automatic assault rifles and rocket-propelled grenades.
But in a confidential report to the UN Security Council, seen by the Reuters news agency, a group of experts says some of these arms shipments are unaccounted for.
A diplomat with knowledge of the current situation in Somalia told the BBC the bombing near the airport suggested that the fight against al-Shabab had stalled. Militants were able to strike at the heart of Mogadishu, he said, because weapons bought by the government were being used in inter-clan warfare rather than fighting against al-Shabab.
Analysis
Last year the United Nations partially lifted an arms embargo, in place since 1992. The move was supported by the US and the UK and was aimed at bolstering the Somali police and army with a limited supply of relatively small weapons such as automatic assault rifles and rocket-propelled grenades.
But in a confidential report to the UN Security Council, seen by the Reuters news agency, a group of experts says some of these arms shipments are unaccounted for.
A diplomat with knowledge of the current situation in Somalia told the BBC the bombing near the airport suggested that the fight against al-Shabab had stalled. Militants were able to strike at the heart of Mogadishu, he said, because weapons bought by the government were being used in inter-clan warfare rather than fighting against al-Shabab.
The al-Qaeda-aligned militants were driven out of Mogadishu more than two years ago but continue to launch attacks.
"A car laden with explosives was remotely exploded in front
of a tea shop just outside the airport," police officer Col Abdikadir
Ahmed told the Reuters news agency.The blast was near a checkpoint at the entrance to the airport complex, the AFP news agency reports.
"At least six people, most of them civilians, died in the car bomb explosion. There are many casualties, serious injuries," police officer Said Mohamed told AFP.
Eyewitnesses say the blast did only superficial damage to the UN vehicles.
Correspondents say there has been a spate of attacks in Mogadishu in the last week, with gunmen targeting Somali army bases in the city, and several artillery shells, fired from about 6km (nearly four miles) away, have landed near the airport.
Residents also report an increase in sustained night-time attacks, with mortar and gunfire, on the northern outskirts of the city.
The airport is the main base for the African Union force which is based in Somalia to help the fledgling UN-backed government gain control of the country from al-Shabab.
The insurgent group has lost control of most main cities in the last two and half years, but it still holds sway in smaller towns and rural areas in southern Somalia.
The airport complex is also where the African Union force is based