A United Nations investigator, Philip Alston, says video footage which appears to show Sri Lankan troops executing captured Tamil Tiger rebels, is authentic.
Several naked men - bound and blindfolded - are shown being shot in the head with semi-automatic weapons.
The ABC reports the Sri Lankan government insists the footage is fake and has repeatedly denied that its forces are guilty of war crimes or human rights breaches.
Mr Alston, a UN special rapporteur, said three independent experts had confirmed the video was authentic, renewing calls for a war crime inquiry.
The government in Colombo said it concluded the video was fake after conducting its own investigation.
The BBC reports it is not clear where the footage, which also shows other bodies on the ground, was taken.
A group called Journalists for Democracy in Sri Lanka said the video was taken in January 2009 with a mobile phone.
Sri Lankan troops finally defeated the Tamil Tigers last May - after nearly 40 years of ethnic warfare that left more than 70,000 people dead.