At least five people were killed and dozens injured when a bomb exploded on a train during rush hour in the Sri Lankan capital on Monday, military officials said.
The military blamed Tamil Tiger rebels for planting the bomb, which detonated in the Colombo suburb of Dehiwala.
The explosion came as the military and the Tamil tiger rebels are locked in a new chapter of a 25-year civil war that has killed more than 70,000 people.
The military defused two powerful bombs on two buses over the weekend and has stepped up security in public areas.
The Liberation Tigers of Tamil Eelam (LTTE), who are fighting for an independent state, were not immediately available for comment but usually deny involvement in such attacks.
The latest blast came as the Sri Lankan military presses an offensive to retake the Tigers' northern stronghold.
Analysts say the military has the upper hand in the latest phase of the long-running war given superior air power, strength of numbers and swathes of terrain captured in the island's east. But they still see no clear winner on the horizon.