A powerful car bomb has exploded in the Somali capital Mogadishu, killing four people, Somali officials say.

Witnesses said the blast was close to the international airport, where African Union troops, UN staff and several Western embassies are based.

Plumes of black smoke were seen in the area and the sounds of gunfire was reported shortly after the explosion.

Security has improved in Mogadishu in recent months, but Islamist group al-Shabab regularly carries out attacks.

A Somali official told the AFP news agency that security forces were alerted to the threat of a car bomb shortly before the blast.

"We had information about this car laden with explosives and we have been following it. But it detonated and four civilians were killed, and the bomber," interior ministry spokesman Mohamed Yusuf said.

Captain Mohamed Hussein, a Somalia police officer, told the AP news agency that the attack targeted a convoy of US-trained Somali intelligence forces, known as the Alpha Group.

The attack comes a week after al-Shabab militants infiltrated the airport compound, killing three AU peacekeepers and a civilian contractor.

Earlier in December, at least six people were also killed when al-Shabab attacked a UN convoy near the airport.

The AU has about 22,000 troops in the country drawn from Burundi, Djibouti, Ethiopia, Kenya, Sierra Leone and Uganda.

Various armed groups have been battling for control of Somalia since the overthrow of President Siad Barre in 1991.