Sri Lankan President Maithripala Sirisena signed the gazette on the OMP yesterday, a move which has drawn praise from the international community.
Farhan Haq, Deputy Spokesman for UN Secretary-General Antonio Gutereres, said that establishment of the OMP was a "significant milestone" for all Sri Lankans still searching for the truth about their missing loved ones.
"The United Nations stands ready to support the process and that the Secretary-General looks forward to the OMP becoming operational as soon as possible, starting with the appointment of the independent commissioners," Haq said quoted Guterres as saying in a statement.
US Ambassador to Sri Lanka Atul Keshap said he was happy about the move as it will provide families waiting for answers hope for swiftest operationalisation of the office.
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Canada's High Commissioner to Sri Lanka Shelley Whiting said the signing of the OMP gazette was a welcome and important step towards providing families of the missing with answers.
The OMP was one of the accountability mechanisms advocated in UN Human Rights Council resolutions on Sri Lanka since 2013.
According to the government estimates, around 20,000 people are still missing due to various conflicts including the 30-year-long separatist war with Lankan Tamils in the north and east which claimed the lives of at least 100,000 people.
The LTTE, which led the separatist war for a separate Tamil homeland, was finally crushed by the Lankan military in 2009 with the death of its supremo Velupillai Prabhakaran.