Russia and Ukraine swapped accusations of breaking a U.S.-brokered ceasefire on Sunday, with both sides claiming to have suffered casualties in drone and artillery strikes over the past 24 hours.

President Trump had announced Friday that Russia and Ukraine would swap 1,000 prisoners as part of a three-day ceasefire, coinciding with the observation of Victory Day in Russia over the weekend. 

On Sunday, Ukraine's President Volodymyr Zelenskyy said that Russia was neither observing the truce nor "even particularly trying to," adding there had been no calm in front line areas despite a lull in large-scale attacks and pledged that Ukraine would retaliate to any aggression shown by Moscow.

"Yesterday and today, Ukraine refrained from long-range retaliatory actions in response to the absence of large-scale Russian attacks," Zelenskyy said in evening statement, stressing Ukraine's increasing ability to hit targets far inside Russia.

"We will continue to respond in the same mirrorlike manner, and if the Russians decide to return to full-scale warfare, our response will be immediate and significant."

Ivan Fedorov, head of Ukraine's southeastern Zaporizhzhia region, said one person had been killed and three others had been injured by Russian artillery and drone attacks in the last 24 hours. Another 16 people were also wounded in attacks across other regions of Ukraine, local officials said.

Russia's Ministry of Defense, meanwhile, accused Kyiv of committing more than 1,000 ceasefire violations, state media reported, citing a daily briefing. The ministry said Ukrainian forces had attacked civilian targets in several Russian regions and carried out strikes against Russian military positions on the front line.

Russia's military had "responded in kind" to the ceasefire violations, the ministry said.

Two people were injured by Ukrainian shelling in the Russian-occupied part of Ukraine's Kherson region, the area's Moscow-installed leader Vladimir Saldo said.

Mr. Trump had said Friday that the break in fighting could be the "beginning of the end" of the war. Both Russia and Ukraine had previously announced ceasefires on different days, so the timeline of the latest three-day pause was somewhat unclear. But Mr. Trump had said the temporary ceasefire would suspend fighting from Saturday through Monday, terms to which Ukraine and Russia both appeared to agree.

Zelenskyy, who had said Russian authorities "fear drones may buzz over Red Square" during the May 9 parade in Moscow, followed up on Mr. Trump's statement by mockingly declaring Red Square temporarily off-limits for Ukrainian strikes to allow the Russian parade to go ahead. The Kremlin shrugged off the comment as a "silly joke."

Russian presidential aide Yuri Ushakov said on Sunday he expects U.S. envoy Steve Witkoff and Mr. Trump's son-in-law, Jared Kushner — who have both taken a leading role in negotiations to end the war — to visit Moscow "soon enough."

However, he stressed that Moscow would not move from its demand that Kyiv's troops withdraw from Ukraine's eastern Donbas region. "Until (Ukraine) takes that step, we can hold several more rounds, dozens of rounds (of negotiations), but we'll be stuck in the same place," Ushakov was cited by the state news agency Tass as saying.

Previous ceasefires, most recently at Orthodox Easter, have failed to produce any tangible results amid deep mistrust between Moscow and Kyiv more than four years after Russia launched its invasion of its neighbor. U.S.-led diplomatic efforts to stop the war have also largely stalled.