The three-member presidential commission was mandated to inquire into and report on alleged abductions or disappearances during the period from June 10, 1990 to May 19, 2009 when the army began the decisive military campaign that crushed the LTTE, ending the nearly 30-year ethnic conflict.
President Mahinda Rajapaksa extended the commission's term to August 12, 2014.
The commission has so far received nearly 16,000 complaints from all parts of the country. The first round of public hearings in Jaffna in the Tamil-dominated north concluded last week.
The US is expected to move a third resolution in as many years at the UNHRC session censuring Sri Lanka on its lack of progress on human rights accountability and reconciliation with the Tamil minority after the civil war ended. Both the earlier resolutions were supported by India.
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The previous two resolutions also committed Sri Lanka to the implementation of the recommendations of its own Lessons Learnt and Reconciliation Commission (LLRC).
A large number of disappearances during the conflict remain a wide human rights concern with international calls for credible mechanisms including an international probe.