Ban launched the investigation "to advise him on the issue of accountability with regards to any alleged violations of international human rights and humanitarian law during the final stages of the conflict in Sri Lanka.
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Spokesman Martin Nesirky said in a statement that the three-strong panel would be chaired by Marzuki Darusman from Indonesia, the UN's special envoy for North Korea, and hoped to complete its work in four months.
Sri Lankan authorities have faced numerous allegations of war crimes during a final military offensive against Tamil rebels in the northeastern part of the island last year that ended the 37-year war.
Videos, photographs and satellite images presented by human rights groups as evidence of war crimes have been rejected as forgeries by Colombo.
Last week Sri Lankan President Mahinda Rajapakse denied his troops fired at a "single civilian" and dismissed calls for a war crimes probe.
"Our troops carried a gun in one hand and a copy of the human righ