Russian forces have taken over Ukraine's second biggest power plant, an adviser to Ukrainian President Volodymyr Zelenskyy said on Wednesday, following an earlier claim by Russian-backed forces to have captured it intact.

Seizing the Soviet-era coal-fired Vuhlehirsk power plant in eastern Ukraine would be Moscow's first strategic gain in more than three weeks in what it calls its "special operation" to demilitarize and "denazify" its neighbor.

"They achieved a tiny tactical advantage — they captured Vuhlehirsk," adviser Oleksiy Arestovych said in an interview posted on YouTube.