The UN agency counts at least 274 civilian deaths and another 1,763 injured. The total number of casualties is thus the highest in a month since spring 2022.
Rocket and drone attacks accounted for just under half, 45 percent, of the deaths - especially in cities far from the Russian border, such as the capital Kyiv.
Since Russia's full-scale invasion of Ukraine in February 2022, more than 16,000 civilians have been killed, according to the UN.
Military losses are usually not reported by Ukraine or Russia, but are believed to be much higher. Overall, hundreds of thousands of soldiers are feared to have been killed in the war.
Correction: In an earlier version of the text, an incorrect comparison was made of how bloody this May was compared to previous months of war.
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