NEW DELHI, OCT. 7 (WEDNESDAY) -- Ethnic Tamil rebels killed dozens of Sinhalese, including eight soldiers they had been holding captive, in an outburst of violence yesterday and early today, reports from Sri Lankan officials said.

Tamils killed 14 Sinhalese in the northern Jaffna Peninsula and at least 25 others in an attack on a village in Eastern Province, the reports said.

The Liberation Tigers of Tamil Eelam said they killed the Jaffna victims in revenge for what they said were the forced suicides of 12 of their men, including two regional leaders. The 12 captured Tamils swallowed cyanide pills Monday as government troops were putting them on a plane for a flight to Colombo, the capital, for interrogation.

{The Press Trust of India reported that Tamil Tiger leader Vellupillai Prabhakaran sent a message to Sri Lankan and Indian officials declaring an end to a July 29 cease-fire, United Press International reported. The message charged that Indian peace-keeping troops had violated the accord by not intervening to save the 12 captured Tamil rebel fighters.}

Indian officials here acknowledged that the latest killings have dealt a severe setback to the efforts of their peace-keeping force in Sri Lanka to bring an end to the country's five years of ethnic violence.

"It will take us weeks to try to get back to where we were," said one Indian official. Other observers said the recent sudden surge of violence, including the shooting death of an Indian soldier late last week, had placed India's peace-keeping effort in a precarious position.

The killing of the eight soldiers was certain to increase unease among many in the Sri Lankan military and ruling party over the July accord to end this nation's ethnic violence. Last night's killings in Erabur village in the Batticaloa district will be likely to sharpen the divide in the already troubled Eastern Province.

Local officials in Erabur said at least 25 people were killed when Tamil guerrillas attacked the town during the night. The toll is expected to increase, according to news agency reports.

There were no signs that India was reconsidering its peacekeeping role, however.

"Neither side really wants us to leave," said one official. "There is a lot of pressure for us to act more forcefully, but that is not our style. Once you shoot an arrow you can't get it back and you can't be sure it will do what you want it to do."

The suicides at Jaffna's Palaly airbase Monday and yesterday's killings of prisoners underscore the militancy of the main Tamil guerrilla group and the difficulties still ahead in forging an accord.

Tamil Tiger guerrillas, many in their teens, long have worn a capsule of cyanide around their necks and have vowed to commit suicide rather than be captured alive. Very few were captured during the five-year-long guerrilla war.

A peace agreement signed by Sri Lankan President Junius Jayewardene and Indian Prime Minister Rajiv Gandhi two months ago had brought hopes of an end to the bloodletting between Sri Lanka's majority Sinhalese and its minority Tamils, which has cost thousands of lives since 1983.

Indian officials say they have gotten Tamil guerrilla groups to surrender much of their weaponry, including most of their machine guns and mortars.

"They no longer have the capability of carrying out guerrilla warfare," said one official.

Yet, a spate of small-arms attacks, many among Tamil groups, has left Sri Lankan authorities nervous.

In addition, the Tamil Tigers, the strongest of the guerrilla groups, had refused to endorse the Gandhi-Jayewardene accord until about a week ago, when they agreed to cooperate in forming a joint governing council for Sri Lanka's Northern and Eastern provinces, as called for under the accord.

A day after agreeing to the makeup of a provisional council, however, the group's leadership withdrew its endorsement, drawing sharp criticism from Colombo and New Delhi. The action also opened the door to the latest violence, which threatens to undermine the agreement.

The surge in violence, before yesterday, was mostly in Eastern Province, which is divided almost equally among Sinhalese, Tamil and Moslem Sri Lankans. Recent attacks there included four on Indian peace-keeping units, apparently by government irregulars or local police.

When 17 Tamil Tigers were taken into custody on a boat off the Jaffna coast, it became a test of the new peace accord.

Sri Lankan authorities insisted that the boat was carrying heavier weapons and was illegal under the agreement. The rebel group insisted its men carried only personal weapons, left tacitly acceptable in the agreement, and said they would commit suicide if the Indian peace-keepers allowed the government to move the men away from the main government base on the Jaffna Peninsula, which also is the main Indian Army headquarters for Sri Lanka.

A Tamil Tiger spokesman said that if the men were allowed to die, the group "will not be bound to observe the cease-fire and cooperate" with the Indian peace-keepers.

According to one report, a Sri Lankan commander was trying to persuade them to move to a waiting plane when Army commandos tried to storm the hut to grab the men before they could break the cyanide capsules. They were unsuccessful and six of the men reportedly died on the spot. Five or six others -- the numbers vary according to different reports -- died later in the base hospital.

According to reports from Colombo, the Tamil Tigers told the Sri Lankan Army yesterday that the bodies of the slain soldiers could be found at the Jaffna bus station. The eight had been captured March 23 near Jaffna town. Their bodies were riddled with bullets, a military spokesman said.

Later yesterday, a Sri Lankan police constable was reported killed in Vavuniya town and the manager and deputy manager of a government-owned cement plant on the Jaffna northern coast were gunned down in their offices.