More than 20 journalists and human rights activists have called for the immediate release of Ukrainian blogger and RFE/RL contributor Stanislav Aseyev, who has been held for more than 18 months by Russia-backed separatists in eastern Ukraine.

The group made the call in a joint letter dated December 12 as its was participating in an annual seminar organized by the Norwegian Helsinki Committee in Sundvolden, near Oslo.

The signatories included the committee’s Secretary-General Bjorn Engesland, American journalist Simon Ostrovsky, Armenian reporter Yuri Manvelian, and Ukrainian human rights defender Svitlana Valko.

Aseyev went missing in Ukraine’s Donetsk region on June 2, 2017, and weeks later Amnesty International said it had received information that the journalist was being held by the separatists who control parts of the region.

Participants at the Anna Politkovskaya Seminar called on the Russian government to “use any and all levers of influence available to it to pressure” the separatists to release him.

They also expressed concern over “recent reports of Aseyev’s treatment at the hands of his captors who have allegedly denied him medicines and have held him in a cold and damp environment.”

“It is also believed that a 'confession' tape that has surfaced in the Russian media in which he admits to espionage was produced under severe duress,” they said in their joint letter.

Aseyev, who writes under the pseudonym Stanislav Vasin, “was the only independent journalist brave enough to continue to live and work in Donetsk when he was apprehended,” the letter also read.

“He frequently reported on daily life in the war-torn city and was known for his even-handed treatment of both the Ukrainian government and pro-Russia groups fighting for control of the area,” it added.

In August, a bipartisan U.S. congressional caucus called for Aseyev's release, describing him as "one of the few independent journalists to remain in the region under separatist control to provide objective reporting.

Ukraine's National Union of Journalists, Reporters Without Borders, the Committee to Protect Journalists, and Amnesty International have all expressed concerns over Aseyev's whereabouts.

More than 10,300 people have been killed since April 2014 in the conflict between Ukrainian forces and the separatists in Donetsk and the neighboring region of Luhansk.