David Cameron calls for an investigation into Sri Lanka allegations as the FCO says "convincing evidence of violations of human rights" were aired in the Channel 4 documentary Killing Fields.
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The film Sri Lanka's Killing Fields documents the final bloody weeks of the Sri Lankan civil war, in which at least 40,000 Tamil civilians were killed. One hour long, it features new evidence of war crimes and crimes against humanity.
The footage shows apparent extra-judicial massacres of prisoners by Sri Lankan government forces, the aftermath of the shelling of civilian hospitals, and the bodies of female Tamil fighters who appear to have been sexually assaulted.
New footage broadcast in the documentary, allegedly captured on a mobile phone by a soldier as a trophy video, shows three people, including one woman, being executed. A man tied to a coconut tree is also killed.
The documentary examines the atrocities carried out by the Tamil Tigers, including the use of human shields. It contains footage depicting the aftermath of a suicide bombing in a government centre for the displaced.
Speaking at Prime Minister's Questions, David Cameron said the documentary referred "to some very worrying events that are alleged to have taken place".
"The Sri Lankan government does need this to be investigated and the UN needs this to be investigated," the prime minister said.
"We need to make sure we get to the bottom of what happened and that lessons are learned."
Earlier, Foreign Office Minister Alistair Burt said he was "shocked" by the "horrific scenes" in the film and urged the Sri Lankan Government to investigate allegations that war crimes were committed.
"The recent UN Panel of Experts' report, this documentary and previously authenticated Channel 4 footage, constitutes convincing evidence of violations of international humanitarian and human rights law. The whole of the international community will expect the Sri Lankans to give a serious and full response to this evidence," Mr Burt said.
"Since the end of the conflict the UK has called for an independent, thorough and credible investigation of the allegations that war crimes were committed during the hostilities and the UK Government expects to see progress by the end of the year. I reiterated this message to the Sri Lankan Foreign Minister on 14 June," he continued.
He added that if the Sri Lankan Government refused to investigate, the international community will "revisit all options available to press the Sri Lankan government to fulfil its obligations".
The Sri Lankan High Commission in London has denied it targeted civilians while crushing Tamil Tiger rebels, but said action would be taken if any allegations of atrocities were proven to be true.
It said images shown in the documentary had not been verified as genuine, and the disturbing footage could cause hatred among Sri Lankan communities.
"The Channel 4 film has the potential to incite hatred amongst different communities in Sri Lanka, including future generations, and thereby adversely affect the ongoing national reconciliation process," the Sri Lankan High Commission said in a statement.
"The malicious allegations in the film regarding the conduct of the Sri Lankan army are in striking contrast to the standards maintained by the Sri Lankan army."
But it added that a panel set up by Colombo, known as the Lessons Learnt and Reconciliation Commission (LLRC), was ready to take note of the claims.
"If the allegations levelled by Channel 4 or any other party are found to be genuine, the LLRC will take due note of all such cases and remedial measures will be taken by way of legal sanctions," the high commission said.
Sri Lanka had previously insisted that there would be no investigation because no war crimes had been committed by its troops while defeating Tamil Tigers by May 2009.
It has refused to allow a three-member UN expert panel to visit the island nation to probe such claims.
Presented by Jon Snow, Sri Lanka's Killing Fields, was shown to the United Nations Human Rights Council at the end of May. The UN screening was attended by a number of ambassadors from nations including the US and UK.
Jon Snow believes the film is "one of the most important" stories he has ever reported, saying:
"It forms a vital part of the evidence of alleged war crimes - crimes, allegations, which the UN itself has called 'credible'. It shows Channel 4 putting a campaign for truth and justice at the very centre of its News and Current Affairs output. And above all, it shines bright unflinching light on a terrible crime - a crime which so far has gone unpunished."
"Whether those responsible are brought to trial at the International Criminal Court, or at the Hague will in part depend upon the pressure from those who see this film upon their own politicians to support the ringing for charges," he added.
Channel 4 News first broadcast the footage allegedly showing government troops executing Tamil prisoners in August 2009.
Last November, a second video of the same massacre emerged, revealing the naked dead bodies of at least seven women. The faces of some of the government troops could also be seen.
Following an investigation, Channel 4 News identified of one of the female victims in the video as a high profile member of the Tamil Tiger communications team. A potential date and location of the massacre was also determined.
A UN special envoy said the footage appeared to be evidence of "serious international crimes".
The Sri Lankan government has consistently rejected the footage as falsified.
As the UN says it will continue its investigations, pressure has increased on the Sri Lankan authorities to allow an international inquiry into allegations that thousands of civilians were killed at the end of the 26-year war.