The assault brought an end to a 37-year-old ethnic conflict that had claimed 100,000 lives since 1972, but hurt the reputation of the country abroad amid criticism of the conduct and tactics of government forces.
The UN estimates that 7,000 civilians perished during the first four months of last year, though this figure is disputed by the government.
On Thursday, UN human rights envoy Philip Alston concluded that video footage said to be of Sri Lankan troops executing unarmed Tamil Tiger rebels last year was authentic and urged Colombo to accept a war crimes probe.
Reacting on Friday, Sri Lankan Human Rights Minister Mahinda Samarasinghe again dismissed the video as a fraud and accused Alston of leading a personal crusade against the country.
"Philip Alston is on a crusade of his own to force an international (war crimes) inquiry against Sri Lanka," Samarasinghe told AFP. "We object to the procedure he followed. He should have shared his information with us first."
He added: "As far as we are concerned, the video is not genuine and it has been doctored."Sri Lanka's Foreign Minister Rohitha Bogollagama questioned the timing of Alston's statement three weeks before Sri Lanka's presidential elections.
Bogollagama said the remarks led to "apprehensions" that he was pursuing a campaign against Sri Lankan authorities with the "deliberate timing to coincide with the current sensitive phase of national elections".
He said Sri Lanka would resist any move against it at the world body.
Sri Lanka has always dismissed calls to probe its rights record, arguing that there are no grounds for an investigation given that there were no civilian casualties or extrajudicial killings.
In May last year, Colombo managed to stave off a resolution condemning it at the UN Human Rights Council thanks to the backing of China.
But new charges emerged last month when the former army chief, Sarath Fonseka, alleged that troops had been ordered by a top defence official to execute surrendering rebels, a charge the government vehemently denies.
The government accused Fonseka, who is challenging President Mahinda Rajapakse in the January 26 vote, of "betraying" the country and making the statement for political gain.
Three Tiger political wing leaders who were arranging their surrender with UN assistance were shot dead on May 17, according to official reports. The government maintained that they were killed by the guerrillas themselves.
Secretary General Ban Ki-moon said, through his spokesman Martin Nesirky in New York, that "a full and impartial investigation into allegations of human rights is critical if we are to confront impunity" and bring justice.
Nesirky said Ban informed Colombo that he was considering appointing a commission of experts "to advise him further and to assist the government in taking measures to address possible violations of human rights and international humanitarian law".
The execution footage that has sparked the latest war of words between Colombo and the UN was shot during the final stages of the army's battle against the Tigers, according to the group that distributed it.
It shows a man dressed in army uniform shooting a naked, bound and blindfolded man in the back of the head, while the bodies of eight others can be seen nearby in a muddy field.
It was not clear if the dead were Tiger rebels or civilians. A 10th man was also shot in the same way towards the end of the video while men in the background gloat over the killings.
Alston said the authenticity of the video was established by three US-based independent, qualified experts he had commissioned to conduct an impartial evaluation after four Sri Lankan specialists concluded it was a fake.