A naked, blindfolded man crouches on the ground, as a uniformed soldier kicks him in the head then abruptly ends his life with a point- blank rifle shot. Other bodies lie nearby, their blood staining the earth around them.

The video, aired by Britain's Channel 4 news, comes with a warning to viewers of "extremely disturbing scenes." It shows alleged killings of unarmed Tamils by Sri Lankan soldiers during their military assault on Tiger-held areas last January.

And it has touched off a new political battle over the Sri Lankan government's actions as it drove the separatist militants – and thousands of Tamil civilians – into a narrow strip of coastal land where the fighters were defeated last May.

"I had to force myself to watch this video," says Harini Sivalingam, policy director of the Toronto-based Canadian Tamil Congress. "But it shows what we have known all along. This is the smoking gun."

The Sri Lankan government strongly denies that. It says no summary executions or other atrocities have taken place, calling the footage a fake and insisting it was "doctored" by pro-Tiger advocates.

"It's a sad day for independent journalism," said Bandula Jayasekara, Sri Lanka's consul general in Toronto. "We have to ask Channel 4 for the proof. After all, people can say they climbed Mount Everest, but where is the proof of that?"

Sri Lankan army spokesman Brig. Udaya Nanayakkara gave a tougher response. "This video has been made to discredit the armed forces," he said, adding it was filmed "at a time when the Tigers, too, were operating dressed in Sri Lankan military uniforms."

Channel 4 correspondent Jonathan Miller, who presented the video Tuesday, said the material was obtained from the press freedom group Journalists for Democracy in Sri Lanka, which got access to it months after the alleged killings, from the cellphone of a soldier at the scene. In an email to the Star, the Berlin-based group said it "stands firmly (by) the authenticity of the clip." It called the government's denials "routine."

The organization said the video was taken at a time when the Sri Lankan government had barred journalists from the conflict zone, but that it shows the "reality of the behaviour of government forces."

The soldiers shown on the video were speaking Sinhala, the language of majority Sinhalese Sri Lankans, it added.

"From the casual nature of the conversations and the fact that it is taking place in an open area and in broad daylight, it can be surmised that these are not extraordinary acts by rogue elements carried out without permission from the top leadership."

In New York, Human Rights Watch's legal and policy director, James Ross, said the original version of the video was more graphic. "These were blindfolded individuals whose hands were tied. The fact that they were naked males makes it unlikely that they were combat deaths." And, he added, the journalists' group that relayed the video is "not a Tamil group, but a multi-ethnic group of exiled journalists. They are new, but well regarded."

Although the video's authenticity could not be conclusively verified, Ross said "it could be a very serious atrocity, and there is a real need for an investigation into this and other possible war crimes."

A UN special rapporteur who has made similar recommendations said the Sri Lankan government has not followed up on its own efforts to probe past allegations of disappearances and executions.

"My report in 2005 already indicated significant numbers of alleged killings, but commissions of inquiry set up by the government have led nowhere," said Philip Alston, a law professor at New York University. "The approach they've taken, of total denial, has been very successful so far."