The Russian defense ministry said its forces have resumed their offensive in Mariupol and Volnovakha, where evacuation corridors had been arranged between Ukraine and Russia.
In a statement to Russian news agency TASS, the ministry said that "not a single civilian was able to leave Mariupol and Volnovakha along the announced security corridors."
"The population of these cities is being held by nationalist formations as human shields. The nationalist battalions took advantage of the silence to regroup and strengthen their positions," said Maj. Gen. Igor Konashenkov, spokesman for the Russian defense ministry.
"Due to the unwillingness of the Ukrainian side to influence the nationalists or to extend the ceasefire, offensive operations have been resumed from 18:00 Moscow time," Konashenkov said.
More background: The Russian defense ministry said earlier on Saturday it would stop bombarding Mariupol and Volnovakha, which have endured days of heavy, indiscriminate shelling.
Residents there have hunkered down in basements without power and with limited supplies of food and water, volunteers gathering information from the ground told CNN.
But just a few hours after the announcement to pause fire, a top Ukrainian regional official accused Russia of breaking its agreement and evacuations were ceased.