Map of Donetsk and Luhansk Ukraine
Donetsk and Luhansk

Ukrainian President Volodymyr Zelenskiy said Saturday that while a prisoner swap was "supposed" to take place tomorrow, the list of individuals whom central Ukrainian authorities and Russia-backed separatists are to exchange had not been finalized. 
 
The notion of an "all-for-all" prisoner exchange gained momentum December 9 during peace talks in Paris among the so-called Normandy Four — Ukraine, Russia, France and Germany — trying to end the five-year conflict. 
 
"There is supposed to be an exchange [of prisoners] tomorrow," Zelenskiy told journalists during a visit for a bridge opening to Ukraine's Ivano-Frankivsk region, according to his official website. 
 
"We look forward to this. The verification of all people is not completed yet," he said. 
 
He called finalizing plans for the exchange the year's most difficult task. 
 
The UNIAN news agency quoted a representative of Donetsk separatists as saying that Kyiv was expected to release 87 people and the separatists 55. 
But there was no official confirmation of any figures. 
 
There were no definitive, publicly available lists of the prisoners that each side is holding. 

Second exchange
 
This would be the second major prisoner exchange in the past four months involving Ukrainians caught up in the conflict. 
 
In the last one, Russia and Ukraine traded a total of 70 prisoners in a move that many regarded as progress in efforts to deescalate a war that has killed more than 13,000 people since Moscow forcibly annexed Crimea and Russia-backed gunmen grabbed swaths of eastern Ukraine, including parts of the Donetsk and Luhansk regions, in 2014. 
 
Russia insists it is not a party to the conflict, despite significant evidence that includes communication with separatist leaders, captured Russians and Russian casualties in the fighting. 
 
A special representative in Ukraine for the Organization for Security and Cooperation in Europe, which has monitors in eastern Ukraine, has mediated talks among the parties and announced Monday that negotiators had "reached an agreement on a mutual release and exchange of conflict-related detainees by the end of the year." 
 
Zelenskiy won the presidency as a political outsider in April, in part by pledging to seek an end to the conflict — which Kyiv and the West blame squarely on Russia — and clean up rampant corruption for post-Soviet Ukraine's 42 million citizens. 
 
The Associated Press and UNIAN contributed to this report.