Artem Kobzar shared a video on Telegram in which he stood in front of a multi-storey residential building destroyed by the missile attack.
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Rescue workers were searching the rubble in the dark for survivors and victims, he said.
The attack on Sumy occurred after one of the heaviest Russian air raids on Ukraine since the war began.
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Russia struck sites across Ukraine overnight with drones, rockets and cruise missiles, with energy facilities as the main targets.