Russia bombed a fire station in Kherson on Friday, hours before a ceasefire announced by Vladimir Putin was due to come into effect.
One firefighter was killed in the attack.
“Russia announces a ceasefire at noon and hurries to strike in the morning. This is some next-level cynicism,” said Anton Gerashchenko, an adviser to the Ukrainian interior ministry, referring to the strike on the fire station.
The Russian president ordered the 36-hour ceasefire for the traditional Christmas holiday which is celebrated by both Ukraine and Russia.
Russian state television declared at midday that the ceasefire had come into effect.
But Russia appeared to break its own ceasefire quickly, with witnesses reporting shelling in the city of Bakhmut hours after the pause to fighting was announced.
Read Nore:Ukraine-Russia war latest: Russia bombs fire station in Ukraine