Colombo, Sri Lanka ? A bomb tore through a packed bus carrying civilians today in central Sri Lanka, killing 20 people and wounding 50 others, the military said.

The blast inside the bus occurred at 7 a.m. in Dambulla, a town about 90 miles northeast of the capital, Colombo, said military spokesman Brig. Udaya Nanayakkara, blaming separatist Tamil Tiger rebels.

The bus traveling to the northcentral town of Anuradhapura was stopped in Dambulla when the blast occurred, Nanayakkara said.

Spokesmen for the rebels could not immediately be reached for comment. But the group, listed as a terror organization by the U.S. and European Union, routinely denies responsibility for such attacks.

Violence has intensified on this Indian Ocean island and more than 700 have been killed after the government withdrew from a cease-fire with Tamil rebels last month.

The bus attack came a day after government troops attacked rebel bunkers along the front lines in the island nation’s embattled north, triggering a battle that killed 10 guerrillas and two soldiers, the military said. Six soldiers were wounded

Army troops, backed by artillery and mortars, launched a pre-dawn attack on rebels guarding the front lines in the northern Jaffna peninsula and destroyed 10 rebel bunkers, said Brig. Udaya Nanayakkara, the military spokesman.

On Jan. 16 a roadside bomb attack by suspected rebels on a bus near the southeastern town of Buttala killed 27 people. Last Tuesday 18 people were killed in a similar bus attack in rebel held north.

Also dozens of other civilians were found hacked and shot to death in the southeast and a mass grave was recovered with 16 bodies killed execution style in a northcentral village.

Both the government and the rebels blame each other for the attacks against the civilians.

The Tamil Tigers have been fighting since 1983 for an independent state in the north and east for the country’s ethnic Tamil minority after decades of being marginalized by Sinhalese-dominated governments. The fighting has killed more than 70,000 people.