Strikes across Ukraine, Russian-occupied territory and Russia over the past day killed at least 16 people, authorities said on Sunday as the 40th anniversary of the Chernobyl nuclear disaster prompted new warnings about risks posed by attacks near the plant during Russia’s more than four-year invasion of its neighbour.

Ukraine’s President Volodymyr Zelensky marked the anniversary with a warning that Russian attacks risk repeating history.

“Russia is once again bringing the world to the brink of...