Russian President Vladimir Putin on Tuesday formally lowered the threshold for his country’s use of nuclear weapons, days after the US allowed Ukraine to attack inside Russia with US missiles.

The Kremlin announced that Putin had approved an updated nuclear doctrine – a document governing how Russia uses its nuclear arsenal – including a statement that Moscow could unleash a nuclear attack if subjected to an attack by a non-nuclear country that has the has the support of a nuclear state.

Russia’s Defense Ministry said Ukraine had carried out its first attack on Russian territory using US-supplied long-range weapons, hitting a military facility in the Bryansk region with an ATACMS missile.

Russian air defenses shot down five ATACMS missiles, but fragments of another “fell on the technical territory of a military facility in the Bryansk region, causing a fire that was quickly extinguished. There were no casualties or damage,” the statement said.

“According to confirmed data, the deployed ATACMS were US-made operational-tactical missiles,” the report said.

Two US officials later confirmed to NBC News that Ukraine had fired ATACMS near the city of Karachev in the Bryansk region, across the country’s northern border, marking the first use of the US-supplied weapons in Russia.

Although Ukraine has already hit targets deep inside Russia, it has used homemade drones and not Western missiles with the firepower of the ATACMs, they said.

The Ukrainian army said it had struck a military arsenal near Karachev in Bryansk. It was not specified which weapons were used in the attack.

The changes to Russia’s nuclear doctrine mark the most significant saber-rattling yet by the Kremlin, which has consistently warned of a possible nuclear war in the 1,000 days since the all-out invasion of Ukraine.

“The update of the nuclear doctrine was necessary to bring the document into line with the current political situation,” Kremlin spokesman Dmitry Peskov told state news agency TASS in a commentary published early Tuesday.

Peskov outlined Moscow’s new threat in light of Washington’s policy change: that the Ukrainian military’s use of Western non-nuclear missiles against Russia under the new doctrine could trigger a nuclear response.

Still, the use of nuclear weapons would be a “last resort,” he added.

A US State Department spokesperson said the Russian change was not surprising.

“Since the beginning of the war of aggression against Ukraine, the country has sought to coerce and intimidate both Ukraine and other countries around the world through irresponsible nuclear rhetoric and behavior,” said Matt Miller of the State Department. “Despite what Russia says, neither the United States nor NATO pose any threat to Russia.”

Putin had announced the update to his country’s policy earlier this year as he sought to warn the West against easing restrictions on Kiev’s use of long-range weapons to strike deep inside Russia.

Russia also retained the option to use the weapons even if Belarus were attacked, he said at the time. And the new doctrine reflects that shift.

“Aggression against the Russian Federation and its allies by a non-nuclear country with the support of a nuclear state will be considered a joint attack,” it said.

The doctrine also says that “the Russian Federation may use nuclear weapons in the event of a critical threat to the sovereignty and territorial integrity of itself and of Belarus,” a shift from previous language stating that it may use nuclear weapons “ when the existence of the state is in danger.”

The changes follow Putin’s warning to the US and its NATO allies that any use of their Ukraine-supplied long-range weapons against Russian territory would mean NATO and Russia are at war.

The Biden administration has long resisted calls from Kiev to ease restrictions on the weapons it has supplied to its ally.

But after the U.S. and others said thousands of North Korean troops had joined the battle alongside the Kremlin’s military, U.S. officials told NBC News that the Biden administration had authorized the use of the long-range ATACMS missile systems for limited attacks in Russia.

The shift drew condemnation from the Kremlin, with Peskov saying on Monday that Washington was adding “fuel to the fire” and provoking “a further escalation of tension around this conflict.”

The changes “create more leeway for a Russian nuclear response to Ukrainian – or, as the Kremlin puts it, Western – attacks on Russian territory,” said Tatiana Stanovaya, a non-resident scholar at the Carnegie Endowment for International Peace and founder and head from the political analysis agency R.Politik.

She pointed to the change in leadership in Washington as a possible motive behind the timing of the updated nuclear doctrine.

“Putin may see the current situation as a strategic ‘interim moment’ – anticipating possible peace initiatives from (President-elect Donald) Trump, while highlighting what he sees as the ‘irresponsibility’ of Biden’s policies. Putin could try to present the West with two stark choices: “Do you want nuclear war? You will get it,” or “Let’s end this war on Russian terms,” Stanovaya said in a post on X.

“This marks an extremely dangerous moment,” she added.

This article was originally published on NBCNews.com