The Office of High Commissioner for Human Rights (OHCHR) was empowered by the UNHRC resolution late March to carry out an independent investigation into alleged rights abuses during Sri Lanka's final stages of the war with the LTTE which ended in 2009.
In the terms of reference for the probe showed on the OHCHR website, it says it will continue to seek to engage with the Government of Sri Lanka in accordance with the resolution.
Sri Lanka has dubbed the resolution an intrusive step taken by the rights body and the appointment of the investigating panel as a breach of its sovereignty.
The government has refused to cooperate with the investigation.
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The UN rights chief in June appointed three experts -- former Finland President Martti Ahtisaari, former High Court judge of New Zealand Silvia Cartwright and former President of the Human Rights Commission of Pakistan Asma Jahangir -- to play a supportive and advisory role.
The OISL will conduct a desk review of existing documents and information, including government and civil society reports, collect and document victims testimonies and the accounts of survivors, witnesses and alleged perpetrators, the website says.