President Mahinda Rajapakse extended the December 31 deadline of the panel he appointed in November to formulate a response to the US report, which accused Sri Lankan forces of war crimes while battling Tamil separatists.
"The president has... extended by four months the period given to the committee to study and report on the US State Department Report," the president's office said in a statement.
A recent query by the United Nations over remarks by the country's former army chief Sarath Fonseka that some surrendering rebels were killed in cold blood was also being referred to the panel for study, the statement said.
Sri Lanka's foreign ministry has already dismissed the US report as "unsubstantiated and devoid of corroborative evidence.
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Sri Lanka has been under international pressure to investigate allegations of human rights abuses and war crimes during the final stages of its battle against the Tamil Tiger rebels, who were defeated in May.
Among claims detailed in th