The whole of Ukraine's western city of Lviv is without power after widespread strikes were reported - including in the capital Kyiv.
Neighbouring country Moldova also reported "massive" blackouts, although it has not been directly hit.
Lviv's mayor urged people to shelter, while the head of the wider Kyiv region said critical infrastructure and homes had been attacked.
Russia has recently increased its attacks on Ukrainian energy networks.
More than half of Moldova is without electricity, deputy prime minister Andrei Spinu wrote on Twitter.
"Massive blackout in Moldova after today's Russian attack on Ukraine's energy infrastructure," he said.
Earlier, an air-raid alert was issued across Ukraine, followed by reports of explosions in a number of locations.
Lviv mayor, Andriy Sadovy, said children had been taken to shelters with their teachers - and urged parents not to pick them up until the alarm was over.
Shortly before the fresh reports from Kyiv and Lviv, officials said southern Ukraine had come under renewed assault.
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