A bomb tore through a packed civilian bus Wednesday in southeastern Sri Lanka, killing 23 people and wounding 67 others, the Sri Lankan military said.

The attack in the remote town of Buttala, about 150 miles southeast of Colombo, came as the government formally withdrew from a tattered cease-fire with Tamil Tiger rebels.

Military spokesman Brig. Udaya Nanayakkara, who provided the casualty figures, said it was unclear if the bomb was on the bus or planted by the roadside.

Senior Superintendent Daya Samaraweera, a local police official, said it was a roadside bomb and that gunshots were fired at the vehicle after the blast.