Un wants an international investigation into the alleged killings

Christof Heyns presented the video footage to the UN human rights council in Geneva, saying a careful technical analysis of the video had led him to the conclusion that it was authentic.

The video was allegedly recorded during the final stages of Sri lanka's conflict with Tamil tiger rebels, which ended in May 2009.

The Sri Lankan government however continues to maintain that the footage is a fake.

Evidence of serious HR violations

The 5 minute video shows graphic images of naked men, hands tied behind their backs, being shot in the head.

UN officials had already seen part of this footage and concluded it was genuine.

Speaking to BBC correspondent in Geneva, Imogen Faulkes, Christof Heyns says that the UN had investigated the images.

“The ballistic expert for an example say, it is the live ammunition being fired. The medical expert analysed the body movements. Two audio video specialists looked in to any tampering of the video footage, which was a very elaborate scientific process. What they say is that there are five sequences, they have changed in sequence around but not been tampered with”.

Sexual violence

The UN's special investigator into extrajudicial killings called in forensic video analysts, a pathologist and a firearms expert to examine an extended version, including footage of soldiers laughing over the bodies of naked women and their conclusion is the same.

“With a video of naked women victims, we need to look at if sexual violence had also taken place” says the UN investigator.

International investigation

Speaking to BBC Christof Heynes said "in this scenario, what we see they are almost definitive kind of example that one would give, of a war crime or crime against humanity".

He wants an international investigation into the alleged killings shown in the video.

“These are images of real people being killed in a very direct way
It will be difficult to ignore such crimes. The whole issue of accountability is important”.

Brutal acts of Tamil Tigers

Sri Lanka maintains the video is a forgery - and government ministers say, the UN should let Sri Lanka conduct its own investigation.

"The video is put together by people who sympathise the Tamil Tigers. No one is presenting the accounts of brutal acts committed by the Tamil Tigers". Laxman Hullugalle, Director of the media centre for National security told the BBC.

"The Tigers killed pregnant women and little children. Many civilians were killed in cold blood". Hulugalle explained.

With more evidence of alleged atrocities due to be shown in Geneva later this week, the pressure is growing on the UN human rights council to debate Sri Lanka, and to investigate the role of the army in the closing months of the civil war.