"Over the past several weeks, Russia has bombed out more than a third of Ukraine's energy system, plunging millions in the cold," Blinken said after a meeting with NATO counterparts in Bucharest.

"These are President Putin's new targets. He's hitting them hard. This brutalisation of Ukraine's people is barbaric."

NATO allies pledged to help Ukraine rebuild its shattered power grid as they met with the country's foreign minster on Tuesday.

Blinken said Western allies were setting up a coordination group to synchronise their support to restore Ukraine's infrastructure.

Blinken insisted that at the same time as they look to help Ukraine repair its grid, the United States and allies are also giving Kyiv anti-air systems "to establish the best possible defence".

"We have to do both, and we are doing both," Blinken said.

Washington's top diplomat said that the United States supported "the need for a just and durable peace" in Ukraine, but Moscow's strikes showed it had no interest in achieving that.

"Russia's savage attacks on Ukrainian civilians are the latest demonstration that President Putin currently has no interest in meaningful diplomacy," Blinken said.

"Short of erasing Ukraine's independence, he will try to force Ukraine into a frozen conflict, lock in his gains, rest and refit his forces and then at some point re-attack again."