The United Nations Assistance Mission for Iraq "condemns in the strongest terms the killing of one of its staff members, who was abducted in April 2015 and whose death was verified only on Monday", it said in a statement.
"The Mission calls on the Iraqi authorities to conduct a thorough and transparent investigation and bring the perpetrators to justice." Amer al-Kaissy, the UN's representative in Diyala province, was seized in the city of Baquba on April 26, 2015, and "is suspected to have been abducted by militias active in the area", it said.
Iraq turned to paramilitary forces dominated by Iran-backed Shiite militias to help counter an offensive by the Islamic State jihadist group that overran swathes of territory in 2014. While militia forces have played a major role in combatting the jihadists, they have also carried out abuses including killings, kidnappings and destruction of property.
UN Iraq representative Jan Kubis said the mission had made repeated unsuccessful appeals to the authorities following Kaissy's abduction.
"For more than nine months we have sought the intervention of the government on the local and national levels to ensure his safe return to his family," Kubis said in the statement.
"We have expressed serious concern about the lack of progress in identifying his whereabouts and achieving his safe release. I am deeply disappointed that our efforts and appeals went unanswered," he said.
The UN suffered one of the worst attacks in its history in Iraq in 2003 when a suicide bomber detonated an explosives-rigged truck at the Canal Hotel in the Iraqi capital. The blast smashed a corner of the building, killing Brazilian UN envoy Sergio Vieira de Mello and 21 others.