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 August 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."

ACF had 'tea break' when killers arrived

ACF said it had been prepared to wait for the outcome of the official investigation but had finally lost hope of Sri Lanka acting to bring those responsible to justice.

"Now that relevant domestic mechanisms have been exhausted, witnesses have been silenced and the internal Sri Lankan investigation has become a farce, ACF considers it to be its moral duty to publicly denounce the perpetrators of this crime."

The organisation said only an independent international investigation could now lead to prosecution of the killers.

The aid workers were in Muttur helping local people recover from the devastating aftermath of the 2004 tsunami. ACF was one of the few NGOs not to have withdrawn its staff from the area as fighting between government forces and the Tigers intensified.

ACF insists it delivered aid impartially to rebel and government-held areas but a suspicion among government forces that many aid workers were pro-rebel seems to have provided the motive for the killings.

According to what ACF regards as credible accounts of the incident, the workers were unarmed and having a tea break when the government forces arrived.

They were forced to kneel and 15 of them were shot in the head on the spot by police officers and a home guard in the presence of naval special forces, according to these accounts. Two others appear to have tried to flee and were shot while doing so.

In the days after the killings, ACF officials attempting to reach Muttur were blocked on four occasions by the military and they were not allowed to collect the bodies until August 7.

ACF on Tuesday put up a series of 10 posters in Paris's busy Odeon metro station to denounce the Muttur attack and other killings of humanitarian workers that have gone unpunished.

The first poster shows a Sri Lankan employee receiving a bullet in the head.

The bullet then continues its course on the other posters, piercing through a bottle of water, a sack of flour and a jar of cooking oil.

"When a humanitarian worker is killed, children who need flour, water and oil are doomed," Penrose told AFP.