The bus was near the town of Ampara, about 220 kilometers (130 miles) east of Colombo, when the blast happened, military spokesman Brig. Prasad Samarasinghe said.

At least 15 people were killed and 10 wounded, said Samarasinghe.

"There is no doubt that this was an attack carried out by (the) LTTE, targeting civilians," he said, referring to the rebels by their official name, the Liberation Tigers of Tamil Eelam.

There was no immediate claim of responsibility for the bombing.

Among the dead were 10 women and two children, Samarasinghe said, adding that all the dead were civilians.

The bus stopped at a military checkpoint and the bomb exploded as passengers started to get off to allow troops to search the vehicle, he said.

Samarasinghe said it was too soon to tell if the blast was detonated by a suicide bomber.

Last week a Tamil rebel suicide bomber killed eight people when he drove an explosive-laden tractor into a military base in the east.

The rebels have generally targeted the military in recent years, including stepped up attacks on government forces at sea. Last week the Tigers launched their first-ever airstrike, bombing an air force base on the outskirts of the capital.

At the same time government forces have been assaulting Tamil positions in the east of the island, overrunning their bases and trying to wrest power in the region from the rebels who control vast swaths in the east and north, where they want to set up an independent homeland.

Ampara was the scene of bitter fighting last month between government forces and the rebels as the army captured several rebel strongholds.

The military was on high alert Monday, before the bombing, to deter retaliatory violence by majority Sinhalese after suspected Tamil rebels killed six ethnic Sinhalese laborers at an orphanage construction site.

Gunmen fatally shot the laborers on Sunday in the eastern Batticaloa district, where they were building an orphanage, Samarasinghe said. He speculated that the rebels may have launched the attack to spur a violent backlash by Sinhalese.

Sri Lanka's ethnic war started when Sinhalese mobs targeted Tamils after 13 Sinhalese soldiers were killed by the Tamils in the north in 1983 in a single major attack against the military.

The rebels have fought the government since then to create an independent homeland for the country's 3.1 million Tamil minority after decades of discrimination by the Sinhalese-dominated state.

The Norwegian-brokered cease-fire signed in 2002 that ended more than two decades of fighting remains intact in name only after violence resumed in late 2005. More than 4,000 people have died since then, though both sides still claim to abide by the agreement.

At least 65,000 people were killed before the cease-fire.