Ukrainian efforts to swap high-profile prisoners with Russia are "ongoing" but will not happen on Friday, officials in Kiev say, rejecting reports that the swaps had taken place.
A statement from the presidency denied that the process had been completed.
Ukraine's prosecutor general had earlier shared a Facebook post suggesting filmmaker Oleg Sentsov and 24 Ukrainian sailors, had been freed.
There was no immediate comment on the swaps from Russia.
But a lawyer for Russian prisoners held in Ukraine said none of the prisoners were flying anywhere on Friday.
Relatives of some of the detainees were reported to be gathering at airports in Kiev.
Rumours that a prisoner swap was imminent spread in recent days after Sentsov was moved to Moscow from a prison in Russia's Arctic north and a Ukrainian court freed a Russian journalist.
But the Ukrainian presidency sought to play down the reports, warning against "misinformation" and urging people not to "play with society's emotions".
A spokesman for Ukraine's SBU security agency then told the BBC that the exchange was not going to take place on Friday.
Russian presidential aide Yuri Ushakov told the Tass news agency only that "interaction on this subject is taking place behind closed doors, but I can of course confirm some progress".
Who are the prisoners?
Sentsov was arrested during a protest against Russia's annexation of Crimea in 2014 and found guilty of plotting terrorist acts.
He denied the charges and his trial was denounced by the US, EU and others as politically motivated.
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.
A lawyer for three Russian prisoners thought to be involved in the swap said no-one had left.
"Nobody is flying anywhere. All Russian citizens as well as Ukrainian citizens, who planned to take part in the prisoner exchange, are either at assembly points, detention centres or wherever they were serving their sentence," Valentyn Rybin said in a video posted on Facebook.
What's the wider picture?
Ukraine's new President, Volodymyr Zelensky, said when he took office earlier this year that ending the conflict with Russian-backed rebels in the east would be his top priority as president.
Mr Zelensky is set to meet President Vladimir Putin as part of a summit next month with the leaders of Germany and France. French President Emmanuel Macron said on Monday he had spoken with the Russian and Ukrainian presidents and said "conditions are ripe" for a meeting.
The Russian presidential aide said progress towards a prisoner release was "very important for the whole atmosphere surrounding a settlement of the Ukrainian crisis".
Fighting in eastern Ukraine has claimed about 13,000 lives since 2014, amid a failed ceasefire deal.
Mr Zelensky is a political novice who was a comedian before taking power. This week, Ukraine's parliament sat for the first time since the vote, with Mr Zelensky picking 35-year-old lawyer Oleksiy Honcharuk as his prime minister.