Colombo - A bomb went off inside a bus in Sri Lanka's capital Colombo Tuesday minutes after it was detected and passengers got off, police said.

Despite the evacuation of the passengers, four people were injured, police said. The explosion, which occurred in the centre of the city, caused extensive damage to the bus.

Police said the conductor of the state-run bus had found the unattended parcel and evicted some 60 passengers before the bomb exploded.

The incident came as government troops stepped up military operations in the north of the island, aimed at capturing areas held by Tamil rebels. (dpa)