Independent experts have ruled video footage appearing to show Sri Lankan troops executing Tamil Tiger rebels is authentic, a UN envoy said.
The video, aired on Britain's Channel 4 last August, "is authentic," said Philip Halston, the UN special rapporteur on extrajudicial, summary or arbitrary executions.
Mr Halston said the conclusion was reached by three independent experts and called for a "genuine and independent investigation" by the Sri Lankan government.
WATCH the Channel 4 report. Warning: this video contains disturbing images
The footage was shot during the final stages of the Sri Lankan army's battle against Tamil Tiger separatists of the Liberation Tigers of Tamil Eelam (LTTE).
Mr Halston said he commissioned US-based experts to conduct an impartial evaluation after four Sri Lankan specialists concluded the video was a fake.
Call for independent inquiry
"The independent experts' analyses also systematically rebutted most of the arguments relied upon by Sri Lanka's experts in support of their contention that the video was faked," he said.
"In light of these conclusions, I call for an independent inquiry to be established to carry out an impartial investigation into war crimes and other violations of international humanitarian and human rights law allegedly committed in Sri Lanka," he added.
Colombo should ask the United Nations to set up an independent commission of inquiry, he state.
A spokesman for UN chief Ban Ki-moon said "a full and impartial investigation into allegations of human rights (violations... in Sri Lanka) is critical if we are to confront impunity and bring the perpetrators of such violations to justice".
He said Ban had informed Colombo that he was considering appointing a commission of experts "to advise him further and to assist the government in taking measures to address possible violations of human rights and international humanitarian law".
Footage 'filmed on mobile phone'
In its original report in August, Channel 4 had said that it could not verify the authenticity of the video which it received from a group called Journalists for Democracy in Sri Lanka.
The group claims the video footage was taken by a soldier using a mobile phone.
The disturbing footage shows a man dressed in army uniform shooting a naked, bound and blindfolded man in the back of the head, while the bodies of eight others can be seen nearby in a muddy field.
Mr Alston said forensic patholigist Daniel Spitz found the footage appeared authentic "especially with respect to the two individuals who are shown being shot in the head at close range".
And he said forensic video analyst Jeff Spivack's checks "found no evidence of breaks in continuity in the video, no additional video layers and no evidence of image manipulation".
"While there are some unexplained elements in the video, there are strong indications of its authenticity," Mr Alston said.
Authorities' 'flawed arguments'
"In addition, most of the arguments relied upon by the government of Sri Lanka to impugn the video have been shown to be flawed."
When it was aired, the video was rejected by the Sri Lankan military as a fake aimed at discrediting the security forces.
Sri Lankan authorities have resisted international calls for a war crimes investigation after the United Nations alleged that more than 7,000 civilians had been killed during the first four months of 2009 alone.
The Tamil rebels were finally vanquished last May after nearly four decades of ethnic bloodshed that left between 80,000 and 100,000 people dead.
The government victory ended the LTTE's four-decade struggle for an independent Tamil homeland, one of Asia's longest-running ethnic conflicts.
