According to the Russian President, it would amount to "nothing less than direct involvement of NATO countries" and "significantly change" the nature of the conflict in Ukraine.

Sir Keir Starmer, who is in the US for a meeting with Joe Biden, underlined that the UK is not 'seeking violence' and that Ukraine is free to defend itself against a war instigated by Russia.

For months, Ukrainian authorities have been appealing for authorisation to launch missiles from the West, such as the long-range US ATACMS and British Storm Shadows, deep into Russian territory.

Amid fears of an escalation, US President Joe Biden has restricted the distance that US-supplied missiles can be fired across the border into Russia.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."

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.

According to the president of Ukraine, Russia has begun a counteroffensive in the Kursk region in an effort to drive out the Ukrainian forces that crossed the border five weeks ago.

The defence ministry of Russia reported that ten communities in Kursk had been retaken by Moscow's soldiers.