President Volodymyr Zelenskyy accused Russia on Tuesday of exploiting a US-backed energy truce to stockpile munitions and using them to attack Ukraine with hundreds of drones and a record number of ballistic missiles.
The overnight attack knocked out heating in cities including the capital Kyiv during freezing temperatures.
"It was a deliberate attack against energy infrastructure, involving a record number of ballistic missiles," Zelenskyy wrote on X, a day after saying Moscow had largely observed the moratorium agreed by the two sides on energy facilities.
Ukrainian officials said Russia launched 450 drones and over 70 missiles in strikes that wounded at least 12 people and struck apartment blocks and energy infrastructure as temperatures hovered around -20 Celsius.
More than 1,000 apartment buildings in Kyiv and nearly 270,000 residents in Ukraine's second city of Kharkiv remained without heating, officials said.
"The goal is obvious: to cause maximum destruction and leave the city without heat in severe cold," wrote Kharkiv Mayor Ihor Terekhov on Telegram, adding that a thermal plant in the city had been badly damaged.
Russia and Ukraine said last week they had halted strikes on each other's energy infrastructure, but they disagreed on the timeframe for the moratorium. The Kremlin said it expired on February 1. Kyiv said it should last until this Friday.
Much of Ukraine has been gripped by a new wave of bitter cold after what Ukrainian experts said was the coldest January in six years.

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