The U.N. Human Rights Council has passed two successive resolutions calling for an inquiry. U.N human rights chief Navi Pillay is expected to submit her findings from a visit to Sri Lanka at the council's next session next March.
Sri Lankan troops in May 2009 defeated Tamil Tiger rebels who had fought since 1983 to create an independent state for the country's ethnic minority Tamils.
The government expelled international aid workers and U.N. staff from the war zone in the last stages of the fighting and blocked independent journalists from covering the war, making it impossible for outsiders to know the extent of civilian deaths.
According to a U.N. report, as many as 40,000 Tamil civilians may have died in the last few months of the fighting, which the government disputes.
For two years after the war, Sri Lanka's government had insisted that not a single civilian was killed. But later in 2011 it acknowledged some civilian deaths and announced a census of the war dead but its results were vague.
New Delhi-based rights group Asian Center for Human Rights criticized the census as a "sham" and said the issue in Sri Lanka was not a lack of statistics but accountability for human rights abuses. It called on the U.N rights council to establish a credible international inquiry into Sri Lanka's human rights.
Government troops were accused of deliberately shelling civilians, hospitals and blocking food and medical aid to hundreds of thousands of people boxed inside a tiny strip of land as the rebels mounted their last stand. The rebels were accused of holding civilians as human shields, killing those escaped their control and recruiting child soldiers.