Ukraine hit a Russian weapons arsenal with US-made ATACMS missiles that it fired across the border for the first time, according to two US officials, in a major escalation on the 1,000th day of war.

The attack comes just two days after the Biden administration gave Kyiv the green light to use the longer-range American weapons against targets inside Russia.

The Russian Defense Ministry said Tuesday that Ukraine fired the US-made missiles into Russia’s Bryansk region.

The attack marks the first time Ukraine has used the longer-range American weapons to strike targets deep inside Russia, and shows that Kyiv has wasted little time in making use of its newly-granted powers.

Ukrainian President Volodymyr Zelensky declined to confirm the attack during a Tuesday news conference but said: “Ukraine has long-range capabilities. Ukraine has long-range drones of its own production. We now have a long ‘Neptune’ (Ukrainian cruise missiles) and not just one. And now we have ATACMS. And we will use all of this.”

At 3:25 a.m. local time (7:25 p.m. ET) Tuesday, Ukraine fired six ballistic missiles at a facility in Bryansk, Russia’s Defense Ministry said. It said that American-made ATACMS missiles had been used in the attack.

Russian air defenses said they shot down five of the missiles and another was damaged. Fragments from the damaged missile fell on the territory of a military facility, causing a fire that has since been extinguished. There were no casualties or damage.

On Sunday, US President Joe Biden authorized Ukraine to use longer-range American missiles inside Russia, ending a months-long prohibition aiming to help Ukraine defend itself while not drastically escalating the conflict.

The decision came at a crucial moment in Russia’s invasion of Ukraine. Russia is probing on the frontlines in Ukraine’s east while pummeling its cities with missile and drone strikes, aiming to disable Ukraine’s power grid and weaponize the freezing temperatures for a third consecutive winter.

Meanwhile, thousands of North Korean troops have been deployed to Russia’s Kursk region, where Ukrainian troops launched a daring counteroffensive in the summer.

In a press briefing Monday, US State Department spokesperson Matthew Miller said the involvement of North Korean troops in the war was “a major escalation by Russia, bringing in an Asian military to a conflict inside Europe.”

The decision to allow the use of the Army Tactical Missile Systems, or ATACMS, inside Russia had been under consideration for months. American officials had been divided on the wisdom of allowing the new capability. Some had concerns about escalating the war, while others worried about dwindling stockpiles of the weapons.

Although Kyiv has repeatedly used Ukrainian-made drones to hit targets in Russia – deeper into the country than Bryansk – Moscow has long maintained that using far-reaching Western weapons would constitute a major escalation.

After the reported missile attack early Tuesday morning, President Vladimir Putin updated Russia’s nuclear doctrine – just two days after Biden’s green light.

Under the updated doctrine, Moscow will consider aggression from any non-nuclear state – but with the participation of a nuclear country – a joint attack on Russia.

However, in more than two-and-a-half years of war, Russia has often rattled the nuclear saber in response to perceived escalation by Ukraine and its allies.

After French President Emmanuel Macron said earlier this year that he would not rule out sending European troops to Ukraine, Putin ordered a tactical nuclear weapons drill in response to what he called “threats” by the West.

For months, Ukrainian President Volodymyr Zelensky had argued that “long-range capabilities for our army” is a key part of his “Victory Plan” for winning the war.

Responding on Sunday evening to reports of Biden’s green-light, Zelensky said: “Strikes are not carried out with words. Such things are not announced. Missiles will speak for themselves.”

CNN’s Natasha Bertrand and Daria Tarasova-Markina contributed to this report.