Christof Heyns, the UN Special Rapporteur on extra judicial, summary or arbitrary executions said he is awaiting Sri Lanka's invitation to him to visit, a statement said.
Heyns said Sri Lanka stands out as the most defining moment when carrying out his mandate in 2015, it said.
"They have accepted some of the other mandates but not my mandate, But I have a standing request to the government to visit Sri Lanka", he said.
As per the UN Human Rights Council resolution of 2014, Sri Lanka was expected to allow visits by UN rapporteurs.
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However, former president Mahinda Rajapaksa denied them permission to visit.
But under Rajapaksa's successor Maithripala Sirisena, Sri Lanka has becoming more amenable to the UN system as last month a team from the UN Working Group on Disappearances visited the island.
The latest October 1 UNHRC resolution seeks to set up a local mechanism to investigate war crimes allegations during the last stages of the civil war with the LTTE that ended in 2009.