Russia bombarded Ukraine’s second-largest city with massive strikes in the early hours of Saturday, its mayor said, one night after Moscow carried out one of the war’s largest aerial assaults on Ukraine.

Officials from Russia and Ukraine wrangled about details of a large prisoner swap agreed to at recent talks in Turkey that had been expected to move ahead this weekend.

Russia struck Ukraine with a thunderous aerial bombardment overnight, further dampening hopes that the warring sides could reach a peace deal anytime soon.

The Ukrainian operation was “grounds for a nuclear attack,” declared Vladmir Solovyov, a firebrand host on Russian state TV, calling for strikes on the Ukrainian presidential office in Kyiv, and beyond.

Former Rep. Chris Stewart, a former fighter pilot, says the U.S. isn’t prepared for this change in modern warfare.

Ukraine’s security service released footage Wednesday of its attack on Russian war planes at multiple airbases across the country. In the span of a few hours on Sunday, nearly a third of Moscow’s strategic bomber fleet was destroyed or damaged with cheaply made drones sneaked into Russian territory, according to Ukrainian officials.

Russia asked the U.N. nuclear watchdog on Friday to mediate between Moscow and Washington to resolve the question of what to do with U.S. nuclear fuel stored at a Ukrainian power plant controlled by Russian forces.

Russia's military released the statement after Ukrainian officials said Russia had launched an intense missile and drone barrage at Kyiv, the Ukrainian capital, in the early hours of Friday and that three people had been killed.