Sri Lanka was granted 18 months by a UNHRC resolution in October 2015 to initiate a credible investigation into the nearly three-decades long civil war with the LTTE.
The foreign ministry said it has sought more time to deliver on accountability mechanism.
"What Sri Lanka will undertake at the current 34th session (of the UNHRC), is a two-year extension of the timeline for fulfilment of commitments made in Resolution 30/1 (in October 2015)," the ministry said.
Earlier this month, the UNHRC had criticised Sri Lanka's "slow" progress in addressing its wartime past and reiterated its earlier call for hybrid court of international and local judges to investigate allegations of rights violations.
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However, Prime Minister Ranil Wickremesinghe rejected the demand, saying it was impractical.
The country's main ethnic Tamil party, the Tamil National Alliance (TNA), has taken a stance that Sri Lanka be given more time by the UN to meet all obligations. However, TNA's rival groups are opposed to giving more time by the UNHRC to implement the 2015 resolution's accountability mechanism.
"TNA leader M A Sumanthiran is not speaking for the suffering Tamils," he told reporters in Jaffna.
The TNA and its moderate group headed by the main opposition leader R Sampanthan and Sumanthiran are accused of trying to appease the majority Sinhala government by giving more time to implement the accountability mechanism.
President Maithripala Sirisena government is opposed to the international hybrid court as the UNHRC has insisted on a credible war crimes probe with foreign judges.