US President Joe Biden denounced an "outrageous" Russian assault on Ukraine's power grid Monday, vowing Moscow would "never succeed" in its ongoing war and pledging continued support to Kyiv.

Russia fired hundreds of drones and missiles at Ukraine earlier Monday, killing at least four people and battering the country's already weakened energy grid, officials there said.

"I condemn, in the strongest possible terms, Russia's continued war against Ukraine and its efforts to plunge the Ukrainian people into darkness," Biden said.

"Russia will never succeed in Ukraine, and the spirit of the Ukrainian people will never be broken."

Moscow's largest attack in weeks on Ukraine triggered widespread blackouts and came after Kyiv claimed new advances in its incursion in Russia's Kursk region.

Since invading in February 2022, Russia has launched repeated large-scale drone and missile attacks on Ukraine, including punishing strikes on energy facilities.

The Russian defense ministry confirmed it hit energy facilities in a statement, claiming that they were being used to aid Ukraine's "military-production complex."

"Ukrainian officials report that this outrageous attack resulted in the deaths of Ukrainian civilians and targeted more than two dozen critical energy sites," Biden said.

The attacks early on Monday also wounded over 20 people across the country, Ukrainian officials said.

Biden's comments followed an earlier condemnation by White House National Security Council spokesman John Kirby which denounced "efforts to plunge the Ukrainian people into darkness as the fall sets upon us."