Ms Pillai says HRC should reconsider its 2009 May verdict on Sri Lanka in light of "efforts to combat impunity worldwide"
Senior UN officials and sections of the international community have called for an international body to investigate alleged rights violations during Sri Lanka's civil war.
Some other countries including China have opposed the call.
Speaking at the UN Human Rights Council (UNHRC) annual session Navi Pillai, UN High Commissioner for Human Rights, said she supported special UN panel's recommendations.
The panel appointed by UN Secretary-General Ban Ki-moon said it found "credible allegations" of a wide range of serious violations of international law committed by both the Sri Lanka security forces and Tamil Tigers in the final stages of the conflict.
"I fully support the recommendation to establish an international mechanism to monitor national investigations and undertake its own as necessary," Ms Pillai said.
"It would be important for the Human Rights Council to reflect on the new information contained in this important report, in light of its previous consideration of Sri Lanka and efforts to combat impunity worldwide."
Minister Mahinda Samarasinghe (R) at a previous UNHRC session
Minister says there is no set international standards dealing with conflicts similar to Sri Lanka
Sri Lanka successfully resisted attempts by Western nations in 2009 May during a special session of the UNHRC in Geneva to investigate alleged war crimes.
UN Special Rapotteur on extra-judicial killings Christof Heyns also supported Ms Pillai's call.
New footage from the final days of the war proves that the war crimes indeed took place in Sri Lanka, he told the session.
He was referring to a five-minute video, obtained by a British television station (Channel 4), apparently shows Sri Lankan soldiers killing blindfolded and naked men and women at close range.
Sri Lankan officials have dismissed the footage as fake.
"Crimes of the highest order"
"What is reflected in the extended video are crimes of the highest order - definitive war crimes," Heyns, said.
Rappotteur Heyns said he reviewed the footage showing the apparent execution of unarmed men and women with technical and forensic experts.
What is reflected in the extended video are crimes of the highest order - definitive war crimes
Christof Heyns on Channel4 video
"The overall conclusion reached by the experts is that the video is authentic and the events reflected in the video footage occurred as depicted," he told the Council.
However, China, Cuba and Pakistan were among the countries that opposed any international intervention on Sri Lanka.
While China said that SL can handle their own matters, Cuba opposed a re-opening of Sri Lanka's case.
Meanwhile, Ireland joined EU, US, UK, and France backing the call for an investigation.
Sri Lanka's plantation minister, meanwhile, categorically rejected the calls for an international probe.
Expressing regret that Ms Pillai's statements "that contain pre-judgements" , Minister Mahinda Samarasinghe says the government has resorted to restorative justice through the Lessons Learnt and Reconciliation Commission (LLRC).
"There is no one set of international panacea that can apply for such a complex set of domestic issues," Minister for Plantation Industries and Special Envoy for Human Rights in Sri Lanka said.
The Lessons Learnt and Reconciliation Commission (LLRC) appointed by President Mahinda Rajapaksa “even before the UNSG's panel,” has started the domestic process, he added.
BBC sinhala
