Russia says the use of US long-range missiles by Ukraine will lead to “an appropriate and tangible” response.
Such an attack inside Russian territory “would represent the direct involvement of the United States and its satellites in hostilities against Russia”, a foreign ministry statement said.
President Joe Biden approved the use of the missiles on targets in Russia in a major change of US policy – two months before he is due to leave the White House.
It is not clear if his successor, President-elect Donald Trump, was consulted or whether he will stick by the decision, having promised to end the Russia-Ukraine war.
Ukraine has had US ATACMS (Army Tactical Missile System) with a range of 300km (190 miles) – as well as French and British Storm Shadow missiles of a similar range – but the Western allies had barred Kyiv from hitting Russia with them.
Biden’s decision to lift that condition is a significant moment in the war, which marks its 1,000th day on Tuesday.
Russia launched a full-scale invasion of Ukraine on 24 February 2022.
Moscow has now intensified attacks on Ukrainian infrastructure as the sides appear to have reached a stalemate on the battlefield.
The US decision also follows the arrival in Russia’s Kursk region – where Ukrainian forces captured and are holding onto a small piece of territory – of more than 10,000 troops from North Korea to help President Vladimir Putin’s forces.
Unconfirmed reports say North Korea may send as many as 100,000 soldiers, in addition to artillery and other weapons to its ally.
Ukraine’s President Volodymyr Zelensky has indicated there may be no formal announcement of the US deal – “the missiles will speak for themselves,” he said on Sunday.
Ukraine may use the ATACMS in Kursk first – in fact some reports suggest the US may have restricted their use there as a signal to North Korea to stop sending aid to Russia and to Moscow itself.
Biden’s approval of the long-range missiles – which may be followed by similar authorisations by the UK and France – is being seen in the West as a way of signalling to the Russian leader that he cannot win the Ukraine war militarily.
Putin has not commented on the latest move.
In September, the Russian leader said the use of such missiles by Ukraine would represent the “direct participation” of Nato countries in the war.
On Monday, Putin’s spokesman said the US was “adding oil to the fire”.
But Jon Finer, US deputy national security adviser, said Washington had made it “clear to the Russians that we would respond” – both to the presence of North Korean forces and the “major escalation” in Russian aerial attacks on infrastructure across Ukraine.
The weekend saw intense Russian attacks against Ukraine’s power grid, causing large-scale blackouts. Several people were killed or injured.
On Monday, a Russian strike on Odesa killed another 10 people and injured nearly 50.
Donald Trump has not reacted to Biden’s decision so far.
He swept to victory on 5 November and will return to the White House on 20 January.
Trump has promised to end US involvement in foreign wars and use the taxpayers’ money to improve the lives of Americans.
He has also said he will end the Ukraine war within 24 hours, but has not given details how.
Zelensky recently said he expected Trump to exert pressure on Ukraine and Russia to agree a peace deal within the next year.