Earlier this week, 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.
In April a 200-page report commissioned by UN Secretary-General Ban Ki-Moon concluded that up to 40,000 civilians may have died in the final push by the government to defeat the Tamil Tigers.
The UN report found credible evidence that both sides of the conflict committed serious war crimes. The report called for an international inquiry - that call has so far has gone unheard.
The Secretary-General said he lacked the authority to personally order an investigation into the mass killings.
A spokesman for Ban Ki-Moon said that without the consent of Sri Lanka's government ,or a decision by the UN Security Council, General Assembly, Human Rights Council or other international body, Ban will not move to set up a formal investigation of the civilian deaths.
UN officials concede that Colombo would never consent to such an investigation of its conduct in the conflict.
Sri Lanka has acknowledged some non-combatants were killed in the war, but says the numbers have been inflated by LTTE supporters.