A prisoner swap between Russia and Ukraine is under way, the president’s office in Ukraine says.
The statement did not indicate who was being swapped, and would only say the process was “ongoing” and had yet to be completed.
But a Ukrainian prosecutor earlier said those being freed include the film-maker Oleg Sentsov and 24 Ukrainian sailors who were detained last year.
There has been no confirmation of the swaps from Russia.
Relatives of some of the detainees are already reported to be gathering at airports in Kiev.
Rumours that a prisoner swap is imminent have spread in recent days as Sentsov was moved to Moscow from a prison in Russia’s Arctic north.
Who are the prisoners?
Sentsov was found guilty of plotting terrorist acts in Crimea after Russia annexed the peninsula in 2014.
The Ukrainian was arrested in 2014 during a protest against the annexation.
Sentsov was given a 20-year jail sentence after prosecutors said he was involved in two attempted arson attacks in the city of Simferopol, ordered by extremist Ukrainian group Right Sector. He denied the charges.
The trial was condemned by the US, EU and other film directors.
Sentsov started a hunger strike in May last year to press Russia to free 64 jailed Ukrainians whom he calls political prisoners. After more than 140 days he abandoned the strike to avoid being force fed.
The sailors detained were seized along with three naval ships as they attempted to pass through the Kerch Strait, the only access to Ukrainian ports on the Sea of Azov.
In May an international tribunal ordered Russia to release the sailors and vessels.
Ukraine and Russia swap prisoners, Ukraine says