Pillay called on Colombo to use the time left before she delivers a widely-anticipated report on the country to the UN Human Rights Council next March "to engage in a credible national process with tangible results, including the successful prosecution of individual perpetrators."
Were that not to happen, she warned in an interim report presented to diplomats in Geneva by her deputy Flavia Pansieri, "the international community will have a duty to establish its own inquiry mechanisms."
Sri Lanka, which maintains it is making great strides in its national reconciliation process, flatly rejected the suggestion an international probe might be needed.
Pillay has "no mandate to make such a claim," Sri Lankan ambassador to the UN, Ravinatha Aryasinha, told the assembly of diplomats, insisting his government had put in place "multiple mechanisms to address accountability."
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Sri Lanka has rejected the charges and maintains that its troops did not kill a single civilian.
Pillay, who conducted a fact-finding trip to Sri Lanka last month, said she had received "little new information about the courts of inquiry appointed by the army and navy to further investigate the allegations of civilian casualties and summary executions" towards the end of the civil war.
"We agree with the High Commissioner that if there is no domestic progress on accountability, the call for an international investigation is warranted," she said.
Pillay also lamented the erosion of rule of law in Sri Lanka, and said the controversial impeachment of the country's chief justice earlier this year had "shaken confidence in the independence of the judiciary and separation of powers in general."
Aryasinha harshly criticised Pillay's report, rejecting her "unsubstantiated claims" that the rule of law and democratic institutions were being undermined, and insisting other charges "lack credibility" or were "baseless and disingenuous.