|WARNING: Some viewers may find some of the scenes disturbing.
Britain's foreign ministry says Sri Lanka must look into new allegations of war crimes, or face possible international action, after a video of the apparent killing of Tamil prisoners by government forces was aired in a UK documentary.
The Channel 4 programme also showed the bodies of female Tamil fighters who appeared to have
been sexually assaulted by government forces.
"I was shocked by the horrific scenes," said Alistair Burt, a minister from the UK's foreign ministry.
Burt said on Wednesday that a failure to probe the claims of human rights abuses at the end of the
26-year war with fighters from the Liberation Tigers of Tamil Eelam (LTTE) could lead to international action against Colombo.
The government of Mahinda Rajapaksa, the Sri Lankan president, says the video material is fabricated.
"Sri Lanka's Killing Fields" was aired on Tuesday and appeared to show the summary execution of Tamil prisoners by government soldiers during the end of the civil war in 2009.
Shot by a mobile phone, it specifically shows the apparent execution of three tied up Tamil prisoners including a woman.
Much of it is filmed by Tamil civilians, herded into so-called "no fire zones", and then allegedly fired upon en masse.
The documentary also cited witnesses describing the aftermath of the alleged shelling of hospitals in designated "safe zones".
Tamil Tiger fighters are also implicated in some of the footage.
The UN estimates that tens of thousands of people died, mostly civilians, in the final weeks of the country's civil war.
Ban Ki-moon, the UN secretary-general, has so far resisted pressure to order an investigation into war crimes.
However, the footage has been shown at the UN Human Rights Council, with Christof Heyns, the UN special investigator into extrajudicial killings in Sri Lanka, calling it evidence of "definitive war crimes".
Donald Gnanakone, the president of Sri Lanka's Tamils of Justice, told Al Jazeera "the United Nations and the United States have a serious obligation which they need to fulfil".
"We are hoping against hope that the United Nations would do its follow-up on the UN panel of experts. The UN Secretary General cannot abdicate his responsibilities and pass it onto the 192 countries. He should take positive action."