Electricity supply has been fully restored in Kyiv after Russia's overnight barrage of missile attacks on Ukraine, Kyiv Mayor Vitaliy Klitschko said in a Telegram post Thursday.

Also, the Zaporizhzhia nuclear power plant is now "receiving electricity for its own needs from the Ukrainian grid after power supply was cut," Russian news agency Interfax reported.

-ABC News' Tatiana Rymarenko and Natalia Shumskaia

Russia's overnight barrage of missiles aimed at civilian infrastructure may have knocked heat out to as much as 40% of Ukrainians, the White House said on Thursday.

Russian President Vladimir Putin is attempting to "brutalize" the people of Ukraine, John Kirby, spokesperson for the White House National Security Council, told ABC News' George Stephanopoulos on "Good Morning America" on Thursday.

"It also appears, George, that they were definitely targeting civilian infrastructure," Kirby said. "I would agree with the Ukrainians. He's just trying to brutalize the Ukrainian people"

Russian forces early on Thursday launched 81 missiles from land and sea, Ukrainian officials said. Eight uncrewed drones were also launched in what officials described as a "massive" attack.

Eleven regions and cities were targeted in an attack that lasted at least seven hours, officials said.

Kirby said on Thursday that the White House expects to see more fighting on the ground in Ukraine for at least the "next four to six months."

"We know that the Russians are attempting to conduct more offensive operations here when the weather gets better," he said.

Ukrainian President Volodymyr Zelenskyy on Thursday said Russian officials had returned "to their miserable tactics" as they launched at least 81 missiles at Ukrainian sites overnight.

"The occupiers can only terrorize civilians. That's all they can do. But it won't help them," he said on Telegram. "They won't avoid responsibility for everything they have done."

He added, "We thank the guardians of our skies and everyone who helps to overcome the consequences of the occupiers' sneaking attacks!"

-ABC News' Yulia Drozd and Joe Simonetti

Waves of missiles and a handful of drones were launched overnight by Russia, targeting energy infrastructure and cities across Ukraine, officials said.

The attack on "critical infrastructure" and civilian targets lasted throughout the night, Verkovna Rada, Ukraine's parliament, said on Twitter. Energy was being gradually restored on Thursday morning, the body said.

Ukraine's parliament and military said at least 81 missiles were fired from several bases. Eight Iranian-made drones were also launched, the military said.

Ukraine destroyed 34 cruise missiles and four drones, military officials said on Facebook.

"Russia's threats only encourage partners to provide long-term assistance to Ukraine," said Yehor Chernev, deputy chairman of the Committee on National Security, Defense and Intelligence.

Russia "will be sentenced as a terrorist state" for its attacks, Ruslan Stefanchuk, Rada's chairperson, said on Twitter.

Secretary of State Antony Blinken has determined that Wall Street Journalist reporter Evan Gershkovich is being wrongfully detained by Russia, according to a statement released Monday afternoon.

Two Americans are now considered to be wrongfully detained by Russia -- Gershkovich and Paul Whelan.

Gershkovich’s case will now be transferred to the Office of the Special Envoy for Hostage Affairs, the U.S. government’s top hostage negotiator.

Gershkovich, a 31-year-old New Jersey native who has lived and worked in Moscow as an accredited journalist for the last six years, was in a restaurant in Yekaterinburg on March 29 when Russia's Federal Security Service arrested him on espionage charges that the Wall Street Journal, his colleagues and the U.S. government have said are absurd.

-ABC News' Shannon K. Crawford