Three people were killed and 42 injured when two bombs exploded on crowded buses in the Sri Lankan capital Colombo late on Tuesday.

The blasts were the latest in a series of bomb attacks on public transport.

No-one has claimed responsibility for any of the bombings, but police said they believed the explosions were the work of the Tamil Tigers.

Two people were killed and 18 injured - four critically - when a bomb ripped through a long-distance bus as it was about to leave the capital at midnight.

Police said the blast blew off the roof of the bus as passengers were boarding and the bus then caught fire.

The wounded included those on the bus and some waiting to get into the vehicle, the police said.

The other bus blast occurred in Wattala, just north of the capital, kiling one person and injuring 15.

Vigilance

Police said they had warned bus crews to be more vigilant for unattended luggage, and passengers were urged to be on the alert.

"Because of difficulties in getting a seat in a crowded bus, passengers are usually more worried about securing a seat," a police official said.

"We are appealing to them to be more vigilant about the surroundings."

Officials said state radio and television have stepped up adverts aimed at minimising the risk of parcel bomb attacks.

The attacks came ahead of plans by Norway to bring the Tigers and the Sri Lankan Government to the negotiating table in a bid to end decades of ethnic bloodletting.

Diplomats had expected an escalation of violence by the Tigers and increased military operations by government forces before the talks as both sides wanted to negotiate from a position of strength.