PARIS (AFP) - France's Action Against Hunger (ACF) on Tuesday accused Sri Lankan security forces of killing 17 aid workers in cold blood and then organising a cover-up of what it described as a "heinous" war crime.

In a report on the bleakest day in its history, the charity said it has proof that Sri Lankan army, navy and police personnel were implicated in the Aug 4, 2006 massacre in the city of Muttur, in the mainly Tamil north of the island.

"In one of the most heinous crimes ever committed against humanitarian workers, the 17 aid workers were lined up, forced to their knees and each shot in the head," ACF said.

The aid workers, 16 ethnic Tamils and one Muslim, were killed as Sri Lankan government forces, then engaged in a civil war against Tamil Tiger separatists, took over Muttur.

Government forces finally triumphed in the conflict in 2009 after decades of fighting characterised by numerous atrocities on both sides.

Defence officials in Colombo on Tuesday dismissed ACF's report and questioned why the charity had "withheld" evidence that could have helped local investigators.

"This is another instance of a pattern which has emerged since the end of the conflict where certain organisations level allegations against the government without providing sufficient details to enable an investigation," military spokesman Brigadier Ruwan Wanigasooriya said.

ACF's report into what happened is entitled The Truth Revealed about the Assassination of 17 Humanitarian Aid Workers in Sri Lanka. It alleges that the killers were protected by the Sri Lankan authorities at the highest levels and describes an official investigation into the killing as a "farce" characterised by the systematic destruction of evidence and multiple irregularities.

"Every day we and other humanitarian organisations work in war zones," said Mike Penrose, Executive Director of ACF-France. "It is paramount that those who do not respect humanitarian aid workers are brought to justice and that these crimes do not go unpunished."