President Mahinda Rajapakse said Friday's bus bomb attack was the work of the rebel Liberation Tigers of Tamil Eelam (LTTE) guerrillas who are fighting a major military campaign in the island's north.
The president in a statement warned that the rebels could resort to further bombings and told residents to be more wary.
The Tigers made no comment on the bus bombing, but on Saturday said the military had mounted a fresh assault against rebel positions in the northeastern Weli Oya region.
The Tigers said they were resisting a "large-scale" offensive by troops while military sources said soldiers were making a fresh advance into rebel-held territory, but there were no immediate details of casualties.
On Friday, a powerful time-bomb ripped through an overcrowded bus, blowing off its roof, as it pulled out of a terminal into rush-hour traffic on the capital's outskirts.
"Two more passengers died in hospital and the number of people remaining in hospital this morning is 64," a police s