"UNSMIL is dismayed by the sentencing of the journalist Ismail Abuzreiba to 15 years imprisonment following a trial by a military tribunal in Benghazi," it said on Twitter on Saturday.
"The detention and trial appear to violate Libya's laws as well as its international obligations on the right to a fair trial" and freedom of expression, it added.
The European Union's ambassador to Libya, Alan Bugeja, wrote on Twitter that he was "extremely concerned" by the sentence, which he said came after the journalist had been held in detention for two years or more.
"I call on the authorities to immediately release him, ensure the respect of his fundamental rights and of freedom of expression," Bugeja said.
Authorities in eastern Libya have not specified the exact nature of the charges faced by Abuzreiba, but, according to local media, he was accused of contact with channels and agencies banned in that part of the country.
Haftar launched an offensive on the capital Tripoli, seat of the internationally recognised Government of National Accord, in April last year.
Troops loyal to the GNA in May pushed Haftar's forces back from the southern outskirts of the capital, before repelling them in early June 2020 as far as Sirte, a northern coastal city that is a gateway between Libya's east and west.
The fighting over the last year or so has killed hundreds, including many civilians. It has also heightened the dangers faced by journalists, who have increasingly been harassed and threatened, leading most to leave the country.
Libya ranks 164 out of 180 countries on Reporters Without Borders (RSF)'s World Press Freedom Index for 2020.
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