Vladimir Putin has warned the U.S. and NATO would be at war with Russia if West lets Ukraine use long-range weapons.
US and British foreign ministers met President Volodymyr Zelensky in Kyiv on Wednesday with the war at a pivotal point.
American officials have been discussing easing restrictions on the use of missiles that could strike deep inside Russia.
Putin said the move from the West would change the very nature of the conflict, and issued a severe threat against giving Zelensky more firepower.
It would mean that NATO countries, the US, European countries, are at war with Russia, he told a state TV reporter.
If thats the case, then taking into account the change of nature of the conflict, we will take the appropriate decisions based on the threats that we will face.
Vladimir Putin has warned the U.S. and NATO would be at war with Russia if West lets Ukraine use long-range weapons
He added that clearing the way for Ukraine to use missiles that can hit Russian targets is a decision on whether NATO countries are directly involved in the conflict or not.
Putins warning came 24 hours before President Joe Biden is set to meet British Prime Minister Keir Starmer at the White House.
Zelensky has put pressure on the West to give him more firepower and air defenses after a series of Russian rocket attacks.
Antony Blinken announced Washington would provide more than $700 million in aid, while foreign secretary David Lammy said Britain would stump up a further £600 million ($781 million) in the latest show of support for Kyiv.
There is no word yet on whether Zelensky will succeed in securing that which he really covets - a green light from the White House and Downing Street to begin launching Western-supplied long-range missiles at targets on Russian soil.
If Ukraines Western allies lift the ban, Kyivs soldiers will soon be able to strike Russian military assets as deep as 300km (190 miles) inside Russia with the likes of the US ATACMS and Anglo-French Storm Shadow missiles - a capability that could significantly impact the course of the conflict.
The Institute for the Study of War, a US think-tank, last month established a list of nearly 250 high value military and paramilitary targets within range of the weapons that could be demolished by Ukraine.
Among the main targets laid out by the ISW include as many as 16 Russian air bases, a slew of brigade and division headquarters, artillery and missile units central to Russias air defence capabilities, and a variety of logistics hubs supplying Vladimir Putins units on the frontlines.
Targeting these sites could cripple Russian logistics, command, and combat support, significantly reducing Moscows offensive capabilities in occupied Ukrainian territory - even if Putins troops redeploy most strategic bombing aircraft further east.
The Kursk nuclear power station and several nuclear weapons stockpiles also pose theoretical targets.
Ukraine has already authored several attacks deep into Russia, including on targets in the capital Moscow, a number of oil refineries and ammunition dumps.
But those strikes have been conducted by kamikaze drones which are considerably limited in their scale and are highly susceptible to Russian jamming and air defence systems.