Ukrainian President Volodymyr Zelenskyy said he had a phone call on Sunday with U.S. President Joe Biden and thanked him for the "unprecedented" help Washington has provided to Ukraine since Russian forces invaded in February.

"I thanked for the unprecedented defense and financial assistance that the USA provides to Ukraine," Zelenskiy said, according to his official Telegram messaging app. "This not only contributes to success on the battlefield, but also supports the stability of the Ukrainian economy.

"We also appreciate the help that the USA is providing to restore Ukraine's energy system."

Ukraine's power grid has been battered since October by successive Russian missile and drone strikes, at times cutting off electricity for millions of civilians in winter.