A bomb ripped through a bus as it stopped at a military checkpoint in eastern Sri Lanka on Monday, killing 16 civilians and wounding 25 others, the military said, blaming separatist Tamil rebels.

The bomb shattered windows and left a bloody trail of debris and flesh, raising the death toll from the last 24 hours of violence in this war-ravaged island to at least 36.

The blast is the latest escalation in a week of violence that has seen the rebels' first air raid, a naval battle, a suicide bombing and government airstrikes, as this tropical island edges toward a resumption of full-scale civil war.

The victims were mostly from the country's Sinhalese ethnic majority, said military spokesman Brig. Prasad Samarasinghe, blaming the attack on the ethnic Tamil rebels.

Insurgents denied the military's accusation.

Monday was a religious holiday for the Buddhist Sinhalese, observed with prayers and visits to the temples that dot the nation.

The bomb exploded as the bus stopped at a checkpoint near the town of Ampara, about 130 miles east of Colombo, and passengers started to get off to allow troops to search the vehicle, the spokesman said.

The explosion occurred in the middle of the bus and left a jagged hole in the roof, said Wasantha Chandrapala, a journalist who arrived at the scene shortly after the attack.

Inside, the wreckage was littered with tattered books, cloths and bags, all covered in blood and bits of flesh, he said.

Sirisena, one of the victims, who identified himself by only one name, said he was seated in the front of the bus.

"The bus stopped at the checkpoint and driver got off. Then, a huge sound came and I became unconscious. When I gained my consciousness, I saw other passengers had fallen around me. They were bleeding."

A Tamil Tiger spokesman, Rasiah Ilanthirayan, denied their involvement in the attack and condemn the killing of innocent civilians.

"We believe that this was carried out by forces who are opposed to us to create a bad name for us when the SAARC meeting is being held in India."

Leaders of the South Asian Association of Regional Cooperation, or SAARC, are meeting in New Delhi, India this week. The group comprises India, Pakistan, Sri Lanka, Nepal, the Maldives, Bhutan, Bangladesh and Afghanistan.

It was not immediately clear if the blast was caused 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 Tamil 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 between government forces and the rebels last month when the army captured several rebel strongholds.

Also Monday, the army said it killed at least 12 rebels fighters in a clash in the northwestern Mannar district late on Sunday.

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.

Tamil rebels have fought the government since 1983 to create an independent homeland for the country's 3.1 million Tamil minority after decades of discrimination.

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.