The Russian president said it would “significantly change” the nature of the Ukraine conflict and amount to “nothing less than direct involvement of NATO countries”.
Ukrainian officials have been pleading for months for permission to fire Western-supplied missiles, including long-range US ATACMS and British Storm Shadows, deep into Russian territory.
US President Joe Biden has limited the distance US-provided missiles can be fired across the border into Russia amid concerns over an escalation.
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However, the issue took on added urgency after Iran recently supplied ballistic missiles to Russia, according to the US.
Speaking to Russian state television on Thursday, Mr Putin warned if Ukraine struck Russian territory with Western-made long-range missiles, “it will mean nothing less than the direct involvement of NATO countries, the United States and European countries in the war in Ukraine”.
He added: “This will be their direct participation, and this, of course, will significantly change the very essence, the very nature of the conflict.
“This will mean that NATO countries, US, European countries are at war with Russia.
“If this is so, then, bearing in mind the change in the very essence of this conflict, we will make appropriate decisions based on the threats that will be created for us”.
He did not elaborate on what those decisions could be, but he has previously raised the option of arming the West’s enemies with Russian weapons to strike Western targets abroad and in June spoke of deploying missiles within striking distance of the United States and its European allies.
Responding to the Russian president’s remarks, Sir Keir Starmer backed Ukraine’s right to defend itself.
“Ukraine has the right to self-defence and we’ve obviously been absolutely fully supportive of Ukraine’s right to self-defence – we’re providing training capability, as you know,” the prime minister said.
“But we don’t seek any conflict with Russia – that’s not our intention in the slightest.
“But they started this conflict and Ukraine’s got a right to self-defence.”
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The issue is likely to be discussed in a meeting between Sir Keir and Mr Biden at the White House on Friday.
Earlier this week, Foreign Secretary David Lammy and US secretary of state Antony Blinken pledged to raise Ukraine’s requests with their respective leaders during a visit to Kyiv.
Mr Lammy and Mr Blinken announced further financial support for Ukraine, including a £600m package from the UK and $717m (£550m) from the US, to meet humanitarian, energy and stabilisation needs.
On Tuesday, Mr Blinken used a press conference with Mr Lammy in London to announce that the US believes the Russian military has received shipments of Iranian short-range Fatah-360 ballistic missiles.
Mr Blinken said Russian troops have been trained on how to deploy the munitions, and warned that Russian commanders “will likely use them within weeks in Ukraine against Ukrainians”.
He also said Mr Biden is “not ruling out” allowing Ukraine to fire missiles deep into Russian territory.
Meanwhile, Russia has launched a counter-offensive in its Kursk region to dislodge Ukraine’s forces who stormed across the border five weeks ago, Ukraine’s president has said.
Russia’s defence ministry said Moscow’s forces had recaptured 10 settlements in Kursk.
