Scores of civilians were killed or injured in heavy fighting that broke out in Sri Lanka's northeast on Thursday when government forces launched artillery and air attacks on Tamil Tiger rebels, the insurgents said.

The Defense Ministry in Colombo said that its military operation had inflicted "huge" losses on the rebels, but a military spokesman said the troops were not targeting civilians and that he was not aware of any civilian casualties.

Three government soldiers were killed and 35 wounded, a hospital doctor said.

The fighting centered around a key reservoir and canal in Trincomalee district, which in recent weeks has seen some of the worst fighting since the country's 2002 cease-fire, with estimates of the number of combatants and civilians killed ranging from about 100 to 300.

A senior rebel official, Seevarathnam Puleedevan, said he had no information on rebel casualties. He said 50 civilians had been killed and 200 wounded by the government artillery and air assault.

"They are attacking us on ground and from air," rebel spokesman Rasiah Ilanthirayan said.

Dr. B.G.M. Costa, a hospital doctor in Kantale, a town in the area, said the bodies of three soldiers had been brought in and 35 others were wounded. Two with critical injuries were flown to Colombo.

Military spokesman Maj. Upali Rajapakse said he was not aware of the civilian casualties, but said the rebels were "known for using civilians as human shields and they place their gun positions around civilian villages."

"The Sri Lankan army does not target civilians," he said.

The rebels closed sluice gates at the reservoir on July 20, cutting water to over 60,000 people living in government-controlled villages, after accusing the government of reneging on a pledge to boost water in rebel-held areas.

The rebels announced on Tuesday that they were reopening the gates, saying that heavy attacks on rebel areas by army troops were hurting civilians, and the military said Wednesday that it had reclaimed control of the waterway.

But the government resumed shelling the area around the reservoir the same day, saying it needed to clear the area of rebels so they would not be able to block the water supply again.

"From dawn today, the Sri Lankan army has launched a full-fledged offensive operation against our territories involving thousands of troops, heavy guns and bombardment," rebel spokesman Ilanthirayan said.

The Tamil Tigers began fighting in 1983 for a separate homeland for the country's 3.2 million Tamils, accusing the 14 million Sinhalese majority of discrimination.

The 2002 cease-fire put a temporary halt to the bloodshed, but the truce has nearly collapsed in recent months. Renewed fighting has killed more than 900 people — half of them civilians — since December, cease-fire monitors say.