The UN human rights office also asked the country to appoint international judges to investigate allegations of rights violations.
"The slow pace of transitional justice in Sri Lanka and the lack of a comprehensive strategy to address accountability for past crimes risk derailing the momentum towards lasting peace, reconciliation and stability," it said.
The remarks came at the launch of a report in Geneva on Sri Lanka's progress in addressing crimes committed during the country's three-decade-long civil war that ended in 2009.
But the rights body did not drop its demand for a hybrid mechanism with foreign judges to probe the allegations.
"Adopt legislation establishing a hybrid court, which should include international judges, defence lawyers, prosecutors and investigators,to investigate allegations of violations and abuses of international human rights law and violations of international humanitarian law,and provide it with the resources necessary to enable it to try those responsible promptly and effectively".
At the same time, the report hailed the government for advancing on constitutional reforms and showcasing some positive developments on the broader human rights agenda.
"Party politics, including the balancing of power between the different constituencies of the coalition in the run-up to constitutional reforms, have contributed to a reluctance to address difficult issues regarding accountability or to clearly articulate a unified position by all parts ofthe government.
"The deployment of foreign judges on the accountability mechanism has presented the government with one such difficult issue.
"Unclear and often contradictory messages have been deliveredon transitional justice mechanisms. Public messaging around transitional justice and reconciliation has been generally confusing and at times contradictory," the report said.
Last month, Sri Lanka said it needed more time to probe the alleged war crimes committed during the civil war and insisted that the proposed mechanism to deliver justice to the victims would be homegrown and not a copy of the South African model.
According to the UN figures, up to 40,000 civilians were killed by the security forces during former president Mahinda Rajapaksa's regime that brought the civil war to an end by defeating the LTTE.
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