At least 18 civilians were killed when a roadside bomb went off on Thursday in Somalia's Lafoole town, southwest of the capital, blowing up a packed mini-bus that was passing by, police said.

There was no immediate claim of responsibility. The al Shabaab militants have frequently launched attacks against security forces and civilians around the country in the past. The group wants to topple the Western-backed government.

"All the 18 people on board the mini-bus are dead and burnt. A remotely controlled bomb along the road exploded," said Abidkadir Mohamed, a police officer at the scene.

Nur Ahmed, who was driving along the same road, said the mini-bus was being escorted by a vehicle carrying troops.

A government truck full of soldiers followed by the minibus overtook him at high speed before he heard a loud explosion, Ahmed said.

"The government car which was probably the target escaped undamaged," he said.