"Another round of exchange of prisoners was planned today with the Russian side," the Coordination Headquarters for the Treatment of Prisoners of War said on the Telegram messaging app. "However, it was cancelled at the last moment at the initiative of the Russian side."

The office of Russian Human Rights Commissioner Tatyana Moskalkova did not immediately respond to Reuters request for comment.

Russia and Ukraine have conducted numerous prisoner swaps - most recently on Jan. 8 - amounting to hundreds of captives each in the course of the war, which is now in its 11th month.

On Saturday Moskalkova said on her Telegram messaging app that Russian soldiers had allegedly reported instances of extortion while in Ukrainian captivity.

There was no immediate response from Ukraine to the allegations. Reuters was not able to independently verify Moskalkova's report.

(Reporting by Ronald Popeski; Writing by Ronald Popeski and Lidia Kelly; Editing by Jonathan Oatis)