The Kremlin has accused the Biden administration of escalating the war in Ukraine by allowing Ukraine to use US long-range missiles to strike Russia.
Spokesman Dmitry Peskov said the move would lead to a rise in tensions and represent a new level of involvement in the conflict.
In a statement released on Monday morning, the Kremlin said it was "obvious" the outgoing Biden administration intends to take steps to add "oil to the fire".
Mr Peskov said: “If such a decision has been taken, it means a whole new spiral of tension and a whole new situation with regard to US involvement in this conflict.”
It came after Ukraine’s President Volodymyr Zelensky said missiles will “speak for themselves”, following reports US President Joe Biden has given the green light for Ukraine to use long-range weapons against Russia.
The decision marks a major US policy shift as Mr Biden is about to leave office.
Speaking in his nightly address on Sunday, Mr Zelensky said: “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.”
The move in the US comes after months of speculation over whether Britain would grant permission for UK-supplied Storm Shadow missiles to be used by Ukraine against targets inside Russia.
Mr Zelensky has been pushing for permission to use Storm Shadow and other Western-supplied weapons to hit Russian airbases and other military sites.
Downing Street offered no comment on the reports regarding the US on Sunday evening.
There has been concern about the level of support the US may continue to give Ukraine when President-elect Donald Trump returns to the White House in January.
Sir Keir Starmer said “we need to double down” on support for Ukraine as the major US policy shift emerged.
The Prime Minister pledged that Ukraine was “top” of his agenda at this week’s G20 summit of world leaders and told reporters that “there’s got to be full support as long as it takes”.
“We need to double down on shoring up our support for Ukraine and that’s top of my agenda for the G20,” the Prime Minister told reporters on Sunday.
Earlier this week, German Chancellor Olaf Scholz spoke to Mr Putin in what was the Russian leader’s first publicly announced conversation with the sitting head of a major Western power in nearly two years.
Asked if he had any plans to make a similar call, Sir Keir said: “It’s a matter for Chancellor Scholz who he speaks to. I have no plans to speak to Putin.”
Speaking to reporters on the way to the G20 summit in Brazil, the Prime Minister added: “We are coming up to the 1,000th day of this conflict on Tuesday.
“That’s 1,000 days of Russian aggression, 1,000 days of huge impact and sacrifice in relation to the Ukrainian people and recently we’ve seen the addition of North Korean troops working with Russians which does have serious implications.
“I think on one hand it shows the desperation of Russia, but it’s got serious implications for European security [...] and for Indo-Pacific security and that’s why I think we need to double down on shoring up our support for Ukraine and that’s top of my agenda for the G20.
“There’s got to be full support as long as it takes and that certainly is top of my agenda, shoring up that further support for Ukraine.”
Russian President Vladimir Putin has previously warned Western nations against green-lighting the use of long-range missiles against his country, saying it would represent the Nato’s “direct participation” in the Ukraine war.
