Russian forces pounded Ukrainian power infrastructure before a new round of peace talks, killing three energy workers and leaving tens of thousands of people without power and heat, officials said on Tuesday.

President Volodymyr Zelenskyy condemned the overnight strikes on 12 Ukrainian regions that came under attack hours before the scheduled start of U.S.-backed trilateral talks involving Kyiv and Moscow in Geneva.

"It was a combined strike, specially calculated to cause as much damage as possible to our energy sector," Zelenskyy wrote on X, calling for diplomacy to be backed by "justice and strength."

Russia, which began its full-scale invasion of Ukraine in February 2022, has frequently carried out attacks on Ukrainian energy facilities this winter that have knocked out power and heating.

Ukraine's deputy energy minister said the three workers were killed when a Russian drone struck their car near the Sloviansk power plant, in a frontline area which Moscow wants Kyiv to cede in exchange for peace.

Power infrastructure supplying Ukraine's strategic Black Sea port city of Odesa suffered "incredibly serious" damage, said private energy company DTEK.

"Repairs will take a long time to restore the equipment to working order," the company said on social media.

Deputy Energy Minister Artem Nekrasov said homes in five regions had suffered power cuts as a result of the strikes, and also reported disruptions to heating supply in Odesa and Sumy, a regional capital in northern Ukraine near the Russian border.

Ukraine's air force said Russia launched nearly 400 drones and 29 missiles. Most were downed, but 13 targets in Ukraine were hit, it added.