A top Russian official has said it could go to war with NATO if Ukraine strikes inside Russia with US weapons.
On Thursday, President Joe Biden partially lifted restrictions on how Ukraine can use military supplies sent by the US.
In response, senior Russian security official Dmitry Medvedev issued a series of warnings to the West.
"Russia regards all long-range weapons used by Ukraine as already being directly controlled by servicemen from NATO countries. This is no military assistance, this is participation in a war against us," Medvedev said.
"And such actions could well become a casus belli [an act that provokes a war]."
He said it would be a "fatal mistake" on the part of the West to think that Russia was not ready to use tactical nuclear weapons against Ukraine - and spoke of the potential to strike unnamed hostile countries with strategic nuclear weapons.
"This is, alas, neither intimidation nor bluffing," said Mr Medvedev.
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He said Moscow's conflict with the West was developing according to the worst case scenario and that "nobody today can rule out the conflict's transition to its final stage".
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The partially-lifted restrictions from the US mean Ukraine can use American weapons to strike inside of Russia but only in defence of Kharkiv.
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However, US officials stressed that Ukraine still cannot use American long-range missiles and other munitions to strike inside Russia offensively.
Ukrainian President Volodymyr Zelenskyy and others have been increasingly vocal that US restrictions meant Ukrainian forces could not defend themselves as Russia intensified attacks around the north-east Kharkiv region.
US weapons may not win the war but they could help stabilise Ukraine
Alex Rossi
International correspondent
The frontlines on Ukraine’s north eastern border around Kharkiv are looking increasingly shaky.
The second city is now in striking distance of Russian artillery.
Washington’s change in policy is limited in scope but it means the defenders will now be able fire US munitions into Russia in this area only, to try and stop Russian forces advancing.
Again, the emphasis is being placed on defensive action rather than offensive operations - but that is a matter of interpretation and will look very different in Kyiv, Washington and, of course, Moscow.
Using American weapons for attacks inside Russia has always been a red line for the Americans.
Since the conflict started, the Kremlin has continuously played up with bellicose rhetoric that such moves could lead to a direct conflict with America - and the spectre of a nuclear war.
But the precarious situation in North East Ukraine and the subsequent appeals from the Zelensky administration appear to have changed the calculation of policy makers in Washington.
It marks a sharp departure from where the Americans were at the beginning of the war.
Then the supply of weapons was only meant to be defensive in nature and to be used inside Ukrainian territory.
Now you have a situation where American guns and rocket launchers can be used to target positions inside a nuclear-armed country and enemy.
Russia’s response has been predictable - it’s warning that the move is a serious escalation and will have serious consequences.
The move, though, is unlikely to change the course of the war.
For many months Ukraine has been striking deeper inside Russian territory and it has not changed Russia’s will to fight.
But it’s hoped the power of American weapons will help Ukraine stabilise an area that has been under constant attack for the last three weeks and looks increasingly weak.
But it also increases the possibly of a direct confrontation with Russia in the future.
A Putin-backed thinktank also suggested on Thursday that Russia should consider a "demonstrative" nuclear blast to scare Ukraine from using Western weapons inside its territory.
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Mr Medvedev, the senior Russian official responsible for Friday's threats, frequently makes dire threats to both Ukraine and the West, often invoking Russia's nuclear weapons.
Some commentators even say his wild statements could be designed to ensure he is not viewed as statesmanlike and therefore not considered a threat to Mr Putin's authority.
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Although many do not attach huge significance to his threats, they could give a flavour of what the Kremlin is thinking, according to diplomats.
