Human Rights Watch on Wednesday urged the UN Human Rights Council to launch "an independent, international investigation into war crimes" committed in the final months of the Sri Lankan conflict.
It said that since the Council adopted a resolution on Sri Lanka in March 2012, Colombo "has taken no significant steps to provide justice for victims of abuse and accountability for those responsible".
"Over the past year the Sri Lankan government has alternated between threatening activists who seek justice and making small, cynical gestures to keep the international community at bay," it said in a letter to members of the Council.
"The Human Rights Council should dismiss these tactics, end the delays and authorize an independent, international investigation into the estimated 40,000 civilian deaths at the conflict's end."
On 11 February, the UN Office of the High Commissioner for Human Rights issued a highly critical report on the Sri Lankan government's failure to provide justice and accountability.
It also urged "an independent and credible international investigation into alleged violations of international human rights and humanitarian law".
Human Rights Watch called on Council members to support a resolution that would establish an international investigation under the high commissioner's office.
India is likely to back a US resolution against Sri Lanka at the United Nations Human Rights Commission on 7 March, reported CNN-IBN.
Several independent bodies have reported credible allegations of war crimes and other serious rights abuses committed by government forces and the Liberation Tigers of Tamil Eelam (LTTE) during the armed conflict that ended in 2009.
"However, the government has taken no significant steps to undertake impartial and credible investigations of these alleged violations," said Human Rights Watch.
It said the Sri Lankan authorities had not reported any criminal prosecutions for serious rights abuses committed during the final years of the conflict.
"Indeed, thus far impunity for these abuses has been total.
"Most disturbingly, an army court of inquiry set up by the government to look into these allegations issued a report Feb 15, fully exonerating the army from any liability for civilian casualties."
On top of the government's failings on justice and accountability, the human rights situation in Sri Lanka has deteriorated since the March 2012 Human Rights Council session, Human Rights Watch said.