A United Nations human rights expert has claimed that gruesome footage from the final days of Sri Lanka's civil war is authentic and proves war crimes took place there.

Christof Heyns, who presented the video footage to the UN Human Rights Council, said that after a careful analysis, he had concluded that it was authentic.

The video was allegedly recorded during the final stages of Sri Lanka's 25-year conflict with Tamil Tiger rebels, which ended in May 2009.

"It''s very rare that you have actual footage of people being killed. This is different from Close Circuit TV. This is trophy footage," the BBC quoted Heyns, as saying.

Heyns said that the video showed "definitive war crimes", believed to have taken place in May 2009 that require both domestic and international proceedings to be launched.

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

Heyns added that the footage was evidence of serious human rights abuse.

The Sri Lankan government has angrily denied his claims. It says that the video is a forgery and that the UN should let Sri Lanka hold its own inquiry.