Witnesses said that the bus was carrying officer workers and school children returning from private classes.

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Timeline: The Sri Lankan conflict

The bus burst into flames after the explosion and the death toll is expected to rise.

Saranga Sadara, who was covered in blood from helping the wounded, said he was in a neighbouring bus when the bomb exploded.

"The whole place smelled of explosives, and debris was all over," he said.
 
No one has claimed responsibility for the bombing.

Civilian deaths

Al Jazeera's Minelle Fernandez, reporting from Colombo, said it has been a violent week in the country.

"Unfortunately this has culminated in many civilian deaths," she said.

"The whole place smelled of explosives, and debris was all over"

Saranga Sadara, witness
Fernandez also said that there had been a wave of attacks since the end of the ceasefire between the LTTE and the Sri Lankan government.

"Even before the truce had officially ended, violations of the agreement were common. Now, both sides have no obligation to this agreement, making the situation considerably worse, reflected through the rising death toll," she said.

A similar blast in February wounded 18 people aboard a bus.
  
In the same month, suspected LTTE fighters used a parcel bomb to blow up a crowded bus in northern Sri Lanka, killing at least 20 people.

The latest blast came as security forces and Tamil Tiger fighters were engaged in fierce clashes in the north of the country, where both sides were reported to have suffered heavy casualties.