COLOMBO, Sri Lanka -- The British government said Wednesday it is shocked by a documentary that purportedly shows Sri Lankan troops committing war crimes, and warned that it will back international action unless the island nation shows progress with an investigation this year.
Foreign Office Minister Alistair Burt said he was "shocked by the horrific scenes" in the documentary aired by Britain's Channel 4 television late Tuesday.
"Since the end of the conflict, the U.K. has called for an independent, thorough and credible investigation of the allegations that war crimes were committed during the hostilities, and the U.K. government expects to see progress by the end of the year," Burt said in a statement.
"If the Sri Lankan government does not respond we will support the international community in revisiting all options available to press the Sri Lankan government to fulfill its obligations," he said.
The documentary appears to show civilian deaths and prisoner executions in the last stages of the country's civil war, which ended in May 2009.
It shows screaming civilians running for cover during a shelling attack, and dead men, women and children lying on a road and inside makeshift hospitals that came under attack.
It also shows three people who appear to be young female rebel fighters sitting on the ground blindfolded with their hands tied behind them. A man gives instructions and another man who appears to be a soldier shoots at their heads, and they slump down.
Channel 4 said the video was obtained from ethnic Tamil civilians and Sri Lankan soldiers, and last month a U.N. expert said it showed "definitive war crimes."
Sri Lanka's government says the video is fake.
"As far as the Channel 4 video clip is concerned, at the very outset we had experts brought in and in accordance with their analysis the authenticity of the footage was in serious doubt," Media Minister Keheliya Rambukwella told The Associated Press.
"This kind of manipulation will not be taken lightly," he said, without explaining what action the government would take.
He also criticized Britain's "unilateral" decision to set a time frame in asking Sri Lanka to show progress in its investigation.
Sri Lanka's embassy in London said the 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."
In a statement, it called the allegations regarding army conduct "malicious," but said if they are proven to be true the government-appointed Lessons Learnt and Reconciliation Commission would take legal action.
A U.N. experts panel reported in April that there are credible reports that both the government and Tamil Tiger rebels committed serious human rights violations -- including possible war crimes and crimes against humanity -- in the last months of the decades-long war. It said tens of thousands of civilians may have been killed, and called for an independent international investigation.
U.N. Secretary-General Ban Ki-moon has said he can initiate an investigation only with the approval of the U.N. General Assembly, Security Council or Human Rights Council.
Sri Lanka counts on the support of Russia and China to extricate itself from any possible international action.
China was among Sri Lanka's allies that defeated a resolution in the U.N. Human Rights Council in 2009 within days after the end of the civil war, calling for an investigation into alleged atrocities committed during the conflict. The countries also pushed through a resolution praising Sri Lanka for defeating the Tamil Tiger rebels.