A leading human rights group has called for an international investigation into the execution-style killing of 17 aid workers in Sri Lanka five years ago.

New York-based Human Rights Watch said Wednesday the Sri Lankan government's failure to bring the killers to justice highlights what it called “a broader lack of will to prosecute soldiers and police for rights abuses.”

The Sri Lankan employees of Paris-based aid agency Action Against Hunger were found dead in August 2006 in the eastern town of Mutur. The group said there is strong evidence security forces were involved in the killings. It urged the United Nations to create an independent probe to make recommendations for prosecuting those responsible for abuses during the final phase of Sri Lanka's 26-year civil war between the government and Tamil Tiger rebels.

The armed conflict ended in May 2009 with the rebels' defeat. A U.N. panel has estimated that thousands of civilians were killed in the final months of the war. The panel also said it had found credible allegations of human rights violations, including possible war crimes, committed by both sides.

But the Colombo government has denied the allegations and said the U.N. report is biased and false. On Monday, the Defense Ministry released its official report on what the army called “its humanitarian mission of 2009.” In it, the ministry acknowledged that it may have been impossible to avoid civilian deaths altogether, given the magnitude of the fighting and the “ruthlessness” of the opponent.

Sri Lanka's own investigative body — the Lessons Learned and Reconciliation Commission — is expected to issue a report later this year. It provides no witness protection to Sri Lankans who offer testimony. The U.N. panel labeled it “inadequate.”