MOSCOW/KYIV — A large-scale prisoner exchange between Russia and Ukraine — the only concrete outcome of peace talks — was thrown into doubt on Saturday after Moscow and Kyiv accused each other of delaying and thwarting the swap.

At talks in Istanbul last week, Kyiv and Moscow agreed to release all wounded soldiers and those aged under 25 who had been captured — more than 1,000 people on each side.

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

Moscow on Saturday accused Ukraine of not turning up to collect the 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 prisoner swap.

The exchange was set to be the largest of the conflict, topping last month's 1,000-for-1,000 swap that was agreed at the 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 remarks, 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.

The spat came as Russia launched a massive aerial attack across Ukraine, heavily targeting the city of Kharkiv overnight and throughout on Saturday.

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 toward halting the fighting.

Agencies via Xinhua