The Sri Lankan military pounded rebel posts in the northern and eastern parts of the country by land, sea and air on Thursday, independent monitors and guerrilla officials said, after a land mine explosion ripped through a crowded passenger bus on Thursday morning in northern Sri Lanka.

[The bombings continued Friday morning, Reuters reported, when jets dropped five bombs on rebel headquarters at Kilinochchi.]

The death toll in the bus bombing rose to 64 late Thursday, with 86 wounded, the most serious attack on civilians since the government and rebels signed a cease-fire four years ago.

The military said only that its forces had taken "deterrent" measures. The government was swift to blame the rebel Liberation Tigers of Tamil Eelam for the attack on the bus, which the rebels promptly denied, pointing at the government.

The Sri Lanka Monitoring Mission, which was set up under the cease-fire and which documents truce violations, said it had not determined who was responsible.

The greater uncertainty was whether the deadly events on Thursday were the beginning of full-scale war. Despite mounting violence, neither side has been willing to explicitly renounce the February 2002 cease-fire. "The substance of the peace process has been completely eroded," was how Jehan Perera of the Colombo-based National Peace Council, an independent research and advocacy group, put it. "Only the outer trappings remain."

Talks in Oslo between the warring parties collapsed last week. The topic of the Norwegian-brokered negotiations was supposed to be the role of European-led truce monitors. The Tamil Tigers pulled out before the talks began, objecting to the composition of the government delegation.

The latest violence follows several months of carnage among Sri Lankan soldiers, Tamil Tiger guerrillas and a breakaway faction in the east known as the Karuna group.

Over the last few months, fighting has emptied villages in the northeast. In April, a bomb exploded in a market in Trincomalee, a port town. The same month, the rebels, known as the L.T.T.E., were accused of trying to assassinate the leader of the Sri Lankan Army in the military headquarters here in the capital. That suicide bombing, which killed nine people and wounded the army commander, Lt. Gen. Sarath Fonseca, and more than 20 others, was followed by airstrikes on rebel posts near Sampur. Since April, 500 people have been killed in the conflict, mostly civilians, the Monitoring Mission said. The toll on Thursday was by far the largest number of civilian deaths since the 2002 truce.

The Tamil Tigers accused the government on Thursday of bombing Kilinochchi, the guerrilla headquarters, as well as Sampur and Mullaittivu, strategic eastern installations for the Tamil naval fleet. The Monitoring Mission reported seeing artillery fire from the Sri Lankan Army and Navy.

"Known L.T.T.E. targets are being taken as a deterrent," the Sri Lankan military spokesman, Brig. Prasad Samarasinghe, said Thursday afternoon, without elaborating.

He said the explosion of the crowded bus at rush hour involved two mines placed side by side and detonated by remote control. The bus was traveling to a market town in the historic Anuradhapura district, about 100 miles north of here. Anuradhapura's border villages have been frequent targets in the war, because the district is at the crossroads between government and rebel-held territories.

Police officers said 15 children were among the 64 dead. Most of the injured, who also included children, were being treated at two Anuradhapura hospitals. Nine with bad head injuries were moved to Colombo.

Calling the attack "a barbaric terrorist act," a government minister accused the Tigers of deliberately selecting the bus. "This was not a mistake," said Keheliya Rabukwella, the policy planning minister.

On the Web site of its peace secretariat, the rebels accused the government of attacking the bus, saying attacks on civilians "cannot be justified under any circumstances."

The European Union put the Tigers on its list of banned terrorist groups last month, following the United States, Britain and India. The Tamil diaspora in Europe is a major fund-raising source for the Tigers.

The State Department spokesman, Sean McCormack, blamed the rebels for the bus attack.

Shimali Senanayake reported from Colombo for this article, and Somini Sengupta from New Delhi.

Copyright 2006 The New York Times Company