Kyiv and Moscow agreed to release all wounded soldiers and those aged under-25 that had been captured, with both sides saying the exchange was set for this weekend.
The POW swap -- to involve more than 1,000 people on each side -- along with an agreement to hand over the bodies of thousands of killed soldiers was the only concrete outcome of a second round of peace talks in Istanbul on Monday.
Russia has rejected Ukraine's calls for an unconditional ceasefire, drawing accusations Moscow has no desire to halt its three-year invasion.
"The Ukrainian side has unexpectedly postponed for an indefinite period, both the acceptance of the bodies and the exchange of prisoners of war," Russia's top negotiator Vladimir Medinsky said on social media.
Medinsky said Russia had brought the bodies of 1,212 killed Ukrainian soldiers to the "exchange area" -- the first of 6,000 to be handed over.
Moscow had also sent a list to Kyiv with the names of 640 POWs to be swapped in the first stage.
The exchange was set to be the largest of the war, topping last month's 1,000-for-1,000 swap that was agreed at a first round of talks in Istanbul.
"We urge Kyiv to strictly adhere to the timetable and all agreements reached, and begin the exchange immediately," Medinsky said.
Kyiv did not immediately respond to the accusation.
After the Istanbul talks, Ukrainian President Volodymyr Zelensky said the exchange would take place this weekend, while Russia said it was ready for Saturday, Sunday or Monday.
- 'Powerful attack' -
The accusation came hours after Russia launched a massive aerial attack across Ukraine, heavily targeting the city of Kharkiv.
At least eight people were killed in the overnight barrage and frontline shelling in total, officials said.
Kharkiv came under "the most powerful attack since the beginning of the full-scale war," Mayor Igor Terekhov said.
At least three people were killed and 17 wounded there after Russia pummelled homes and apartment blocks.
Another six were still unaccounted for as of Saturday afternoon, believed to have been in an industrial site that was hit, the local prosecutor's office said.
Three people were killed in the frontline Donetsk region, which has seen the most intense fighting of the war, and a couple were killed in the southern city of Kherson, another city close to the front.
Since Russia invaded in February 2022, tens of thousands have been killed, with millions forced to flee their homes as cities and villages across eastern Ukraine have been destroyed.
Ukraine's Foreign Minister Andriy Sybiga called for Kyiv's Western allies to punish Russia for refusing to halt its invasion.
"To put an end to Russia's killing and destruction, more pressure on Moscow is required, as are more steps to strengthen Ukraine," he said on social media.
The Ukrainian air force said Russia had fired 206 drones and nine missiles in the overnight barrage.
Russia's defence ministry said it had launched a "group strike" against "military-industrial" facilities in Ukraine, adding that all the "targets" had been hit.
The Kremlin in recent days has vowed revenge for a brazen Ukrainian drone attack last Sunday on its nuclear-capable bomber fleet, thousands of kilometres behind the front lines.
And on Friday it called the Ukraine war an "existential question" for Russia.
- Ceasefire hopes dim -
The comments are Moscow's latest to dampen hopes for a breakthrough amid the flurry of diplomacy, as well as telephone calls between Putin and US President Donald Trump.
Despite Trump urging a swift end to the fighting, he Kremlin chief has issued a host of sweeping demands on Ukraine as preconditions to a truce.
They include completely pulling troops out of four regions claimed by Russia, but which its army does not fully control, an end to Western military support and a ban on Ukraine joining NATO.
Ukrainian President Volodymyr Zelensky has dismissed the demands as old ultimatums, questioned the purpose of more such talks and called for a summit to be attended by him, Putin and Trump.
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