Footage broadcast by Channel 4 News showing the alleged massacre of Tamil prisoners is evidence of "serious international crimes", the UN says, as pressure mounts for an international investigation.

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A UN envoy has urged the international community and Sri Lanka to further investigate footage obtained by Channel 4 News purporting to show the summary executions of naked and bound men and women during the country's civil war.

Channel 4 News first broadcast footage of the alleged execution of Tamil prisoners by Sri Lankan soldiers in 2009. Last December, damning new footage came to light of the same alleged massacre and the behaviour of government troops.

The new footage revealed the naked dead bodies of at least seven women and a number of men. The faces of some government soldiers allegedly carrying out the executions could also be seen.

Following an extensive investigation, Channel 4 News uncovered the identification of one of the victims captured in the video - a high profile member of the Tamil Tigers' press and communications wing.

The Sri Lankan authorities have vehemently rejected both videos as fake.

Sri Lanka execution footage: UN calls for investigation

On Tuesday Christof Heyns - the UN's special rapporteur on extrajudicial, summary or arbitrary executions - told a press conference the case should go to the next level of international investigation after earlier recognising the video showed evidence of "serious international crimes".

He stopped short of saying there could be an international war crimes case, but his comments raised pressure on Sri Lanka to submit to an international inquiry into charges that war crimes were committed at the end of its brutal 25-year war with guerrillas of the Liberation Tigers of Tamil Eelam (LTTE).

"The prima facie case should go to the next level of investigation on a domestic and an international level," Heyns said.

"We should recognise the domestic process, but I think in parallel there should be an international investigation."

Sri Lanka has acknowledged some non-combatants were killed in the war, but says the numbers have been inflated by LTTE supporters.

Sri Lanka has also accused the international community of trying to pre-empt its own Lessons Learned and Reconciliation Commission (LLRC).

In dialogue with the UN rapporteur, Mohan Pieris, Sri Lanka's attorney general, said the LLRC had also been studying the video.

He argued that the publication of the videos and "subsequent steps" were "tainted with the fundamental vice of bias and partiality".

Channel 4 News' hour-long investigation, "Sri Lanka's Killing Fields", will be shown in Geneva on Friday on the sidelines of the Human Rights Council and broadcast in Britain on June 14.

It includes the roughly five minutes of footage examined by UN-appointed experts.