A UN statement publicly releasing a report by a world body panel said that Secretary General has been advised that he needs Sri Lankan government consent or a decision by member states in an international forum to go ahead to order an international investigation into allegations of possible war crime.
The statement did not specify the forum and whether it meant that an international probe had to be ratified by the UN Security Council, General Assembly or Human Rights Council.
Ban's statement came as the panel called on the Secretary General to immediately set up "an independent international mechanism" to investigate "credible" allegations that both Sri Lankan government and Tamil Tigers committed serious human rights violations, including some that could amount to war crimes and crimes against humanity, in the months before the decades old civil war ended in 2009.
The three-member panel which collected evidence for over 10 months said tens of thousands died in just five months in the last phase of the war. [Report of Secretary General's panel over Sri Lanka]
"Most civilian casualties in the final phases of the war were caused by government shelling. The government systematically shelled hospitals on all fronts. The government systematically deprived people in conflict zone of humanitarian aid in the form of food and medical supplies, particularly surgical supplies, adding to their suffering," said the report. The UN panel estimated that 40,000 people were killed in the war.
"The panel's determination of credible allegations reveals a very different version of the final stages of the war than that maintained to this day by the government," it said.
The UN said Sri Lankan government had not yet responded to its offer to publish the government response along with the report.
The report also documents alleged human rights violations committed by the Tamil Tigers including using civilians as human shields.
A leading human rights group declared that UN panel's findings show the need for an international investigation.
Brad Adams, Asia Director of the New York-based Human Rights Watch Group said an independent investigation should be set-up. He blamed Russia and China who were blocking efforts to form such a commission.
The US also called for the Sri Lankan government to respond constructively to the report.
"The report highlights the need for an independent and full accounting of the facts in order to ensure that allegations of abuse are addressed and impunity for human rights violations is avoided," Susan Rice, US envoy to the UN said in a statement.
"We strongly support the Secretary General's call for the Sri Lankan authorities to respond constructively to the report and underscore our belief that accountability and reconciliation are inextricably linked," she added.
The UN also said that Ban "regrets the inflammatory tone of some of the recent public statements emanating from Sri Lanka."