An international aid agency suspended its operations in Sri Lanka following the slaying of 15 Tamil aid workers in the northeast, where heavy renewed fighting threatens to shatter the country's already fragile cease-fire.

The aid workers — 11 men and four women — were doing post-tsunami relief for Action Against Hunger in the seaside town of Muttur.

The Paris-based agency, confirming the killing, said it was suspending its operations in Sri Lanka.

"The humanitarian values that are defended and promoted everywhere by Action Against Hunger ... have been scorned," the group's president, Denis Metzger, said in Paris. "This act undermines ... the very status of humanitarian work."

CountryWatch: Sri Lanka

The government did not immediately comment on the slayings.

The aid workers' bodies were found late Friday, Action Against Hunger's local official Eric Fort said. The bodies had bullet wounds and most of them were laying face down, other officials at the agency said, speaking on condition of anonymity.

Amnesty International urged the government to seek international assistance to ensure justice for the killings.

"While we welcome the government's statement that this investigation will be treated as a matter of high priority, we are concerned that other high-profile cases of killings and 'disappearances' remain unresolved months and even years after the crime," Natalie Hill, deputy Asia director at Amnesty International, said in a statement.

Separately, the head of anti-terrorism commando training, Upul Seneviratne, was killed by a bomb near the central city of Kandy, said Inspector General of Police Chandra Fernando, blaming the Tamil Tigers.

"By all accounts ... he was killed by the terrorists," Fernando said.

The latest violence started July 20 when the rebels in the northeast blocked water supplies to some 60,000 people in government-held villages. They said the move came in retaliation for the government reneging on a deal to boost water supplies in rebel areas.

The rebels said they beat back a military offensive early Monday and retained control of the sluice gates in Trincomalee district. Military spokesman, Maj. Upali Rajapakse, rebuffed that claim and said government forces were still fighting to recapture control of the water supply point Monday.

No casualty figure was immediately available.

The Liberation Tigers of Tamil Eelam, as the Tigers are formally known, have fought for more than two decades to carve out a separate homeland for Sri Lanka's 3.2 million minority Tamils in the north and east.

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

The water crisis dramatically escalated the violence, with six days of fighting around the northeastern port town of Trincomalee and the nearby town of Muttur.