Ukrainian President Volodymyr Zelenskiy has been pleading with Kyiv's allies for months to let Ukraine fire Western missiles including long-range US ATACMS and British Storm Shadows deep into Russian territory to limit Moscow's ability to launch attacks.
In some of his most hawkish comments on the subject yet, Putin said such a move would drag the countries supplying Kyiv with long-range missiles directly into the war since satellite targeting data and the actual programming of the missiles' flight paths would have to be done by NATO military personnel because Kyiv did not have the capabilities itself.
"So this is not a question of allowing the Ukrainian regime to strike Russia with these weapons or not. It is a question of deciding whether or not NATO countries are directly involved in a military conflict," Putin told Russian state TV on Thursday.
"If this decision is taken, it will mean nothing less than the direct involvement of NATO countries, the United States and European countries in the war in Ukraine.
"This will be their direct participation, and this, of course, will significantly change the very essence, the very nature of the conflict."
Russia would be forced to take what Putin called "appropriate decisions" based on the new threats.
He did not spell out what those measures could be, but he has spoken of the option of arming the West's enemies with Russian weapons to strike Western targets abroad and in June spoke of deploying conventional missiles within striking distance of the United States and its European allies.
Russia, the world's largest nuclear power, is also revising its nuclear doctrine - the circumstances in which Moscow would use nuclear weapons - and Putin is being pressed by an influential foreign policy hawk to change it to state Russia's willingness to use nuclear arms against countries that "support NATO aggression in Ukraine".
Russia is also holding major naval exercises with China and considering curbs on major commodity exports.
The West casts its discussion around whether it should allow Kyiv to use its long-range weapons to strike Russia as part of an answer to what it says is an escalation of the war by Moscow, which is says has received ballistic missiles from Iran.
Tehran said the claims were "ugly propaganda".
Russia invaded Ukraine in 2022 with tens of thousands of troops, triggering the biggest confrontation between Russia and the West since the depths of the Cold War.
Putin casts the conflict as part of an existential battle with a declining and decadent West he says humiliated Russia after the Berlin Wall fell in 1989 by encroaching on what he considers Moscow's sphere of influence, including Ukraine.
The West and Ukraine describe the invasion as an imperial-style land grab and have vowed to defeat Russia on the battlefield.
