At least 18 civilians were killed when a roadside bomb went off in Somalia, close to the capital, blowing up a packed minibus, police said.

The blast on Thursday in Lafole, which is around 20 kms west of Mogadishu, struck as the bus was passing by. There was no immediate claim of responsibility.

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

Nur Ahmed, who was driving along the same road, told the Reuters news agency that the minibus was being escorted by a lorry carrying government troops. The government vehicle escaped undamaged, he said.

Sources confirmed to Al Jazeera that the minibus was being closely followed by government forces.

Al Jazeera's Mohammed Adow, reporting from Kenya's Nairobi, said: "They [government officials] think it was the forces that were being targeted in the roadside bomb."

The explosion comes less than a week after an attack on a hotel by the al-Shabab armed group in Mogadishu killed at least 15 people.

The group, which wants to topple the government, frequently attacks security forces and civilians.