Ukrainian President Volodymyr Zelensky has condemned a "horrific" Russian ballistic missile attack on the northeastern Ukrainian city of Sumy on Sunday, after repeated warnings from Ukrainian officials that a fresh offensive on the area could be imminent.

A Russian ballistic missile strike on the northeastern Ukrainian city of Sumy killed at least 20 people on Sunday morning, the city's acting mayor said.

Ukraine's air defenses shot down 56 of 88 Russian drones, its air force said. It added that 24 drones were "lost" as the military used electronic warfare to redirect them.

Russia launched a bevy of drone attacks at Ukraine Saturday, killing one person and injuring several others, coming amid warnings that Moscow is replenishing troops for renewed offensives.