Sri Lankan Army soldiers stand guard at the site of an explosion as the wreckage of an exploded bus is seen in the background in Colombo, Sri Lanka, yesterday.--Xinhua/AFP

At least four civilians were slightly injured yesterday afternoon when a civilian bus was exploded by suspected Tamil Tiger rebels in Sri Lankan capital Colombo, defense officials said.

Officials from the Media Center for National Security said the explosion happened inside a bus around 1:50 p.m. (0820 GMT) in central Colombo.

Four civilians suffered minor injuries in the blast and no suspect has been arrested, the officials said, adding that the bus was seriously damaged.

Passengers immediately got off the bus and reached safety distance after an unattended parcel was found inside the bus and the parcel exploded within minutes, said the officials.

The military said the explosion was another attempt by Tamil Tiger rebels to cause mayhem in the island's capital when they are facing fierce assaults by the security forces in the northern Kilinochchi and Mullaittivu districts.