The UN secretary general has set up a panel to look into alleged human rights abuses during the final stages of Sri Lanka's civil war in 2009.
Ban Ki-moon's spokesman said the three-man panel would advise on how to deal with alleged perpetrators.
The UN says more than 7,000 civilians died in the last five months of the conflict between the Sri Lankan troops and Tamil Tiger rebels.
Rights groups accuse both sides of war crimes - a claim which has been denied.
The rights groups have produced what they say is photographic and video evidence of the army executing Tamil rebels, and testimony of indiscriminate bombing of civilians.
The insurgents are accused of using civilians as human shields and killing those who tried to leave the conflict zone.
The 37-year conflict ended last year with the defeat of the Tamil Tigers. The UN estimates 100,000 people were killed.
UN spokesman Martin Nesirky said that the UN panel would be chaired by former Indonesian attorney general Marzuki Darusman.
The other two members are South African human rights lawyer Yasmin Sooka and Steven Ratner, a US international law expert.
Mr Nesirky said the three men "will advise him [Mr Ban] on the issue of accountability with regard to any alleged violations of international human rights and humanitarian law during the final stages of the conflict in Sri Lanka".
However, the panel is not an investigative body and its mandate is vague, which reflects the extreme sensitivity of the subject, says the BBC's Barbara Plett at the UN headquarters in New York.
Neither the UN Security Council nor the UN's Human Rights Council has called for an inquiry, and the panel may be a way for the secretary general to keep up the pressure for accountability, our correspondent adds.
The authorities in Colombo have strongly rejected persistent allegations of human rights abuses.
They have established their own investigation, and have lashed out at Mr Ban's decision.