A suicide attacker rammed the explosives-laden vehicle into a security checkpoint near the airport, local police said.
Security forces were checking cars near the main base of peacemakers African Union (AU) when the bomb when off.
The blast was said to be so powerful it blew off the roofs of nearby houses.
Police believe Islamic extremist group Al-Shabaab are behind the attack, according to Reuters.
Dramatic eyewitness pictures posted to Twitter show plumes of white smoke from the blast streaming into the air.
Eyewitnesses said the blast was followed by heavy gunfire, according to reports.
Police officer Mohamed Ahmed told Reuters: “So far we know three of the security forces died.
“We believe Al-Shabaab is behind the attack.”
“So far we know three of the security forces died.”
Police officer Mohamed Ahmed
The checkpoint is close to United Nations offices and the Peace Hotel, which is popular with foreigners and diplomats.
Unconfirmed reports on social media suggest foreigners staying at the hotel have been evacuated to Halane Amisom basecamp.
There are fears the death toll is likely to rise due to the power of the blast.
There have been no immediate claims of responsibilty for the attack.
However, Al-Qaida linked terror group Al-Shabaab have been known to carry out such attack such attacks.
The group claimed responsibilty for a car bomb attack outside an airport in December which killed 29 people.
In July heavy explosions set off by Al-Shabaab militants killed seven people outside the entrance to the African Union’s main base.
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