Kyiv, Ukraine - A large-scale prisoner exchange between Russia and Ukraine – the only concrete outcome of peace talks – was in doubt Saturday after Moscow and Kyiv accused each other of delaying and thwarting the swap.

The hold-up to an exchange that both sides said would take place this weekend came hours after Moscow's army launched a barrage of missiles, drones, and bombs across the country.

At talks in Istanbul on Monday, Kyiv and Moscow agreed to release all wounded soldiers and those aged under-25 that had been captured – more than 1,000 people on each side.

Russia said it would also hand back 6,000 killed Ukrainian soldiers.

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Moscow on Saturday accused Ukraine of not turning up to collect the dead bodies and not agreeing on a date to swap the captured soldiers, while Kyiv said Russia was playing "dirty games" by not sticking to the agreed parameters for the exchange.

"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.

A defense ministry spokesman said "the Ukrainian side is still refraining from setting a date" for the first stage of the POW swap.

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.

After the Istanbul talks, Ukrainian President Volodymyr Zelensky said it would take place this weekend, while Russia said it was ready for Saturday, Sunday, or Monday.

Responding to Russia's accusations, Ukraine's Coordination Headquarters for the Treatment of Prisoners of War said no date had been agreed for the return of bodies.

It also said a list of names Russia said would be released did not match the terms of the agreement.

"Unfortunately, instead of constructive dialogue, we are again faced with manipulations," it said in a statement on social media.

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"We call on the Russian side to stop playing dirty games and return to constructive work to bring people back to both sides and to clearly implement the agreement in the coming days," it added.

The spat 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, with three people killed and 17 wounded as homes and apartment blocks were pummelled.

Three people were also 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 defense ministry said it had launched a "group strike" against "military-industrial" facilities in Ukraine.

Despite talks, the two sides have made no progress towards halting the fighting.

Russian President Vladimir Putin 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.

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.