A leading human rights group is calling 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 says 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. Human Rights Watch said Wednesday the killings followed fighting between Sri Lankan government forces and separatist Tamil Tiger rebels for control of the town.

HRW 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 Sri Lankan armed conflict, including the case of the aid workers.

Sri Lanka's 25-year civil war ended in 2009 with the military's defeat of the Tamil Tigers. A U.N. panel has estimated that thousands of civilians were killed in the final phase of the conflict. Earlier this year, the panel said it had found credible allegations of human rights violations, including possible war crimes, committed by both the military and the rebels.

The government has denied the allegations. But a defense ministry report released Monday acknowledged for the first time that civilians may have been killed during the last stage of the war. The report said although the military followed a “zero civilian casualty policy,” it was impossible to avoid such casualties given the magnitude of the fighting and the “ruthlessness” of the opponent.