Western analysts and officials say Ukraine has in recent months recorded battlefield successes against Russia's bigger army, disrupting a spring offensive started by Russia amid improving weather.
Russian President Vladimir Putin ordered the ceasefire on Thursday, more than a week after Ukrainian President Volodymyr Zelensky first made the proposal.
Russia's invasion of Ukraine, launched more than four years ago, has cost hundreds of thousands of lives and displaced millions in the bloodiest conflict in Europe since World War II.