Kremlin top spokesman Dmitry Peskov told reporters that this would unleash a "significant new round of escalation" in the war if it proves true, but he also cautioned that Moscow authorities had yet to see official confirmation directly from the White House.

"If such a decision has indeed been formulated and communicated to the Kyiv regime, it marks a significant escalation and a fundamentally new stage in terms of U.S. involvement in this conflict," Peskov said.

The reports center on Biden giving the greenlight for Kiev forces to deploy the Army Tactical Missile Systems (ATACMS) against targets deep in Russian territory.

Peskov further reiterated that the question of Western-backed long-range strikes was "articulated extremely clearly and unambiguously" by President Vladimir Putin earlier this year.

Putin had laid out in September that it would change the very nature of the conflict, and that "it would mean that NATO countries, the US, European countries, are at war with Russia."

Ukraine's president, Volodymyr Zelensky has appeared to confirm the new authorization from Washington. He said that proof will ultimately emerge on the battlefield.

"Today, there's a lot of talk in the media about us receiving permission for respective actions. But strikes are not carried out with words. Such things are not announced. Missiles will speak for themselves. They certainly will," Zelensky said.

He further emphasized that the "long-range capabilities for our army" is a central part of his "Victory Plan" for winning the war, though many analysts, even some who are ardently pro-Kiev, see this new greenlight as too little, too late - and as having no real ground impact on the war (only risking runaway escalation with the West). Reuters has also said, based on Ukrainian sources, that striking Russia with long-range missiles is likely to happen soon, consistent with Zelensky's fresh remarks.