The Sri Lankan military attacked Tamil Tiger rebels from land and air Thursday and the rebels retaliated in heavy fighting that killed at least 13 combatants, officials said. A Nordic cease-fire monitor warned the situation was worsening.

A military spokesman said he was unaware of any civilian casualties, but a senior rebel official, Seevarathnam Puleedevan, said at least 50 civilians had been killed and 200 injured in the government military operation.

"The peace process is in grave danger," Puleedevan said. He appealed to the International Committee of the Red Cross to help evacuate civilian casualties, saying there weren't enough facilities locally available there to treat them. But ICRC official Davide Vignati said the organization had yet to receive an official request from either side.

CountryWatch: Sri Lanka

At least six government soldiers were killed and 49 wounded and hospitalized, a doctor said. A Tiger spokesman, Rasiah Ilanthirayan, said seven rebels had died and 15 others were wounded.

"It is a warlike situation, our forces are retaliating with artillery fire and direct gun fire," Ilanthirayan said.

"The situation is worsening," said Robert Nilsson, a member of the Sri Lanka Monitoring Mission. "Hopefully there will be a change of heart, this has already gone too far."

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.

Dr. B.G.M. Costa, a hospital doctor in Kantale, a town in the area, said the bodies of five 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 any 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, the cease-fire monitors say.