Both sides have been accused of committing abuses against civilians
The UN's high commissioner for human rights has called for an independent investigation into alleged atrocities by both sides in Sri Lanka's civil war.
Navi Pillay said it was the only way to build a sustainable peace in Sri Lanka.
Her comments to the UN's Human Rights Council (UNHRC) come amid growing concern for more than 250,000 civilians now living in government-run camps.
Sri Lanka has rejected Ms Pillay's demands and has called instead for financial aid to rebuild the country.
In her opening speech to the UNHRC's emergency summit in Geneva, Ms Pillay said there were "strong reasons to believe that both sides have grossly disregarded the fundamental principle of the inviolability of civilians".
She said an "independent and credible international investigation" should be carried out to establish "the occurrence, nature and scale of violations of international human rights and international humanitarian law, as well as specific responsibilities".
"Establishing the facts is crucial to set the record straight regarding the conduct of all parties in the conflict," she said.
"Victims and survivors have a right to justice and remedies."
Her comments were echoed by the UN's Under Secretary for Humanitarian Affairs John Holmes, who told the BBC an investigation was essential if the country was to move forward.
Mr Holmes said the Tamil Tigers (LTTE) had used civilians as human shields "in the most cynical and brutal way", but that civilians had also been "affected very badly" by being caught up in army shelling.
UN High Commissioner for Human Rights, Navi Pillay, calls for an investigation
The rights council's special session is considering two different draft resolutions.
One put forward by 17 European and Latin American countries calls on the Sri Lankan government to investigate allegations of rights abuse and for aid agencies to have greater access to displaced people.
But Sri Lanka has drafted its own document calling for UN co-operation in providing humanitarian aid.
Its resolution cites the "principle of non-interference" in states' internal matters and says it is addressing the needs of civilians.
Sri Lanka's ambassador to the UN, Dayan Jayatilleka, said it was "outrageous" to suggest that the government should be investigated.
The country's resolution has been supported by India, whose representative to the council said his country had "serious reservations about the objectives and usefulness" of the session.
A Gopinanthan said the focus of the international community should be on promoting reconciliation and healing in Sri Lanka.
The BBC's Imogen Foulkes in Geneva says it appears that the widespread allegations of violations are not being seriously addressed at the meeting, much to the frustration of rights groups and Ms Pillay.
The end result of the session is likely to be a bland and consensual resolution which does not demand much action from anyone, says our correspondent.
Ms Pillay has also called for journalists and human rights monitors to be given access to the camps for internally displaced people.
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