A meeting between Mr. Putin and Mr. Zelensky would have been the first between the two men since before Russia invaded Ukraine three years ago, launching a war that by conservative estimates has resulted in well over one million dead and injured soldiers on both sides.

As the clock approached the time the Kremlin said its guns would fall silent for three days, the men at the National Guard monitoring center in eastern Ukraine had absolutely no faith in Russian President Vladimir Putin’s ceasefire.

Russia resumed mass drone attacks in Ukraine early on Sunday, after its self-declared three-day pause expired.