The UN refugee agency told Ukraine on Wednesday that extraditing Tajikistan's former prime minister Abdumalik Abdullajanov to his homeland would break international law, and called for the US asylee to be freed.
Abdullajanov, who was granted political asylum in the United States and has lived there for 16 years, was detained last month at Kiev airport as he arrived in Ukraine on a private visit.
Tajikistan has said it will seek the extradition of Abdullajanov, who is accused of trying to stage a coup against President Emomali Rakhmon in the late 1990s.
Last Friday, a Ukrainian court approved an extension of his detention for 12 months.
UNHCR said it was "deeply concerned" about his continued detention and the prospect of his "refoulement" -- the international legal term for unjustly sending a refugee home.
"The former Tajik prime minister has been recognised as a refugee by the United States and had travelled to Ukraine on a valid US-issued travel document with a Ukrainian visa," it said.
"Refoulement is a violation of the 1951 UN Refugee Convention, to which Ukraine is a signatory, as well as a fundamental breach of international human rights law. Refoulement is also prohibited under the national laws of Ukraine," it added.
In addition, since returning Abdullajanov would violate both international and domestic law, his continued detention for extradition purposes lacks a legitimate purpose, it said.
"UNHCR urges the Ukrainian authorities to act in accordance with international law by respecting Mr Abdullajanov's refugee status and releasing him from detention," it said.
Abdullajanov was prime minister from 1992-1993 and in 1994 unsuccessfully stood in elections against Rakhmon. He is seen as having close contact with the opposition outside Tajikistan.
Rakhmon, who has headed the Central Asian state since 1992, is up for re-election in polls expected in November this year.