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Russian President Vladimir Putin has warned NATO leaders that lifting restrictions on Ukraine’s use of long-range Western missiles to launch attacks deep inside his country would be considered an act of war.

“This means that NATO countries – the United States and European countries – are at war with Russia. And if that is the case, then, given the changed nature of the conflict, we will make appropriate decisions in response to the threats that will be directed against us,” Putin told reporters on Thursday.

Putin’s comments come at a time when the United States and its NATO partners appear increasingly open to the possibility of allowing Ukrainian forces to use Western-sourced long-range weapons systems to attack Russian military targets, a possibility that Kyiv has been open about in the war well into its second year.

During a recent visit to Kyiv, US Secretary of State Antony Blinken made the strongest suggestion yet that the White House would consider lifting the restrictions as part of a broader strategic realignment among NATO partners.

“As you heard me say, we have adapted from day one as the needs and the battlefield have changed, and I have no doubt that we will continue to do so as the situation evolves,” Blinken said, speaking alongside Ukrainian Foreign Minister Andrii Sybiha and British Foreign Secretary David Lammy.

British Prime Minister Keir Starmer arrived in Washington on Friday for talks with US President Joe Biden, which are expected to focus on the use of Western weapons against targets in Russia.

Although the US has adjusted its policy to allow limited cross-border attacks on Russia using US-made weapons, US President Joe Biden has not yet authorized the use of longer-range systems. US officials have previously expressed concerns that allowing Ukraine to strike deep into Russia could escalate the conflict.

Before these talks, the Russian secret service FSB had withdrawn the accreditation of six British diplomats in Moscow and accused them of espionage without providing any evidence. Britain later stated that the expulsion had already taken place last month and called the allegations “completely baseless”.

Biden is coming under pressure domestically from lawmakers of both parties to ease restrictions as Ukraine struggles to consolidate its battlefield advances and risks being pushed back by Russian forces.

On Tuesday, the bipartisan Ukraine Caucus in Congress called on Biden to allow Ukraine to attack targets inside Russia with long-range weapons.

A group of key Republicans in the House of Representatives also wrote a letter to the president this week ahead of Blinken’s trip to Ukraine, joining Ukrainian President Volodymyr Zelensky’s calls to lift the restrictions.

But Putin on Thursday expressed doubts that Kiev was capable of carrying out long-range strikes on its own, saying: “The Ukrainian army is not able to use state-of-the-art, high-precision long-range systems from the West” without NATO support in targeting.

The U.S. military already provides intelligence to Ukraine and has assisted with targeting in the past, but not with the long-range systems currently being considered.

Depending on the target, Ukrainian forces may also have other intelligence resources at their disposal, including commercial satellite imagery, according to Dr. Stacie Pettyjohn, senior fellow at the Center for a New American Security.

In a press conference on Thursday, Blinken reiterated that the U.S. is providing intelligence to Ukrainian forces as part of its continued military assistance, but he declined to say whether the U.S. would increase its intelligence sharing.

Asked about concerns about escalation, Blinken said on Wednesday that they were a factor, but “certainly not the only one, and they are not necessarily a decisive factor.”

He also accused Russia of escalation by acquiring Iranian ballistic missiles.

“We are now seeing Russia acquiring ballistic missiles from Iran, which will further escalate its aggression in Ukraine. So if anyone is taking an escalatory action, it appears to be Mr. Putin and Russia,” Blinken said.

The United States delivered long-range Army Tactical Missile System (ATACMS) missiles, which have a maximum range of about 290 kilometers, to Ukraine for the first time in October 2023. Kyiv has long been calling on its Western backers to allow the deployment of longer-range weapons systems on Russian territory.

In a recent interview with CNN’s Alex Marquardt, Ukrainian Defense Minister Rustem Umerov said the airfields Russia uses to attack Ukrainian cities are within range of deep strikes. So far, Ukraine has used its existing stockpile of long-range ATACMS to attack valuable Russian assets in occupied Crimea, including air defense systems, ammunition depots and airfields.

If NATO countries authorize the use of Western weapons for long-range strikes on Russian territory, this technology could also be used to attack short-range ballistic missile (CRBM) systems that have entered the country from Iran in recent weeks, former U.S. Ambassador William Courtney, now an adjunct senior fellow at the RAND Corporation, told CNN. With a range of just 120 kilometers, a U.S.-made system like ATACMS “could hit anything at that distance and well beyond,” he told CNN.

This story has been updated with additional details.

Michael Callahan, Natasha Bertrand, Oren Liebermann and Lex Harvey contributed reporting.