(Reuters) -Ukrainian President Volodymyr Zelenskiy vowed on Sunday to press on with prisoner exchanges with Russia and said any failure by Moscow to uphold humanitarian accords cast doubt over U.S. and other efforts to end the more than three-year-old conflict.

Russia claims Ukraine indefinitely postponed the release while Kyiv maintains it has been delayed until next week.

Russian officials say that Moscow is still awaiting confirmation from Kyiv on a planned exchange of 6,000 bodies of soldiers killed in action and is accusing Ukraine of delaying the process

Massive Russian strikes on Ukraine’s second-largest city killed at least five people on Saturday, officials said, one night after Moscow carried out one of the war’s largest aerial assaults on Ukraine.

Russia on Saturday accused Ukraine of postponing a large-scale exchange of captured soldiers, hours after Moscow's army launched a barrage of missiles, drones and bombs across the country.