US President Joe Biden's national security adviser said on Sunday that Russia's offensive in the Kharkiv region in northeastern Ukraine had "stalled."

"The momentum of this operation in Kharkiv has stalled," Jake Sullivan told CBS. “Kharkov is still under threat, but the Russians have failed to make significant progress in the area in recent days.”

Washington recently gave Kyiv the green light to use US-supplied weapons to defend the Kharkov region, despite concerns that authorizing such strikes could drag NATO into direct conflict with Russia.

"From the president's perspective, it was common sense," Sullivan said. “It just didn’t make sense to not allow the Ukrainians to fire across that border, to hit the Russian guns and emplacements that were shooting at the Ukrainians.”

Biden, who is visiting France, met with Ukrainian President Volodymyr Zelensky in Paris on Friday, reiterating his support for the war-torn country.

Speaking on Saturday, Biden said Russian President Vladimir Putin "won't stop" on Ukraine.

“All of Europe will be under threat. We're not going to let that happen,” Biden told reporters alongside French President Emmanuel Macron after talks at the Elysee Palace.

Russian troops have made their biggest territorial gains in 18 months since launching a surprise ground attack on the Kharkiv region in early May, capturing several Ukrainian border villages and forcing thousands of people from their homes.