Russia vowed retaliation Tuesday after Ukraine used longer-range U.S. missiles to strike its territory for the first time, with President Vladimir Putin issuing a nuclear warning on the war's 1,000th day.

A senior official told AFP that a strike on Russia's Bryansk region earlier Tuesday "was carried out by ATACMS missiles" – a reference to the U.S.-supplied Army Tactical Missile System.

Speaking 1,000 days after Russia invaded Ukraine, Russian Foreign Minister Sergei Lavrov said the attack showed Western countries wanted to "escalate" the conflict.

"We will be taking this as a qualitatively new phase of the Western war against Russia. And we will react accordingly," Lavrov told a press conference at the G-20 summit in Brazil.

Putin signed a decree Tuesday lowering the threshold for using nuclear weapons, a move that the White House, U.K. and European Union condemned as "irresponsible."

He has used nuclear rhetoric throughout the conflict but has grown increasingly belligerent since last year, pulling out of a nuclear test ban treaty and a key arms reduction agreement with the U.S.

Ukrainian leader Volodymyr Zelenskyy accused G-20 leaders in Brazil of failing to act over Putin's nuclear threats, saying the Russian leader had no interest in peace.

He later warned that Ukraine would lose the war if the United States cut military funding to Kyiv.

U.S. President-elect Donald Trump is a vocal skeptic of the billions that the administration of Joe Biden has given to Ukraine since the Russian invasion began in 2022.

"If they cut, we will – I think we will lose," Zelenskyy said in an interview with U.S. network Fox News.

"We will fight. We have our production, but it's not enough to prevail," he added.

Nuclear saber-rattling

Washington this week said it had cleared Ukraine to use ATACMS against military targets inside Russia – a long-standing Ukrainian request.

The United States will also soon provide Kyiv with antipersonnel land mines to shore up its defenses against Russian forces, a U.S. official said late Tuesday.

The official said Washington has sought commitments from Ukraine to use the mines in its own territory and only in areas that are not populated in order to decrease the risk they pose to civilians.

Russia said Tuesday that Ukraine had used the ATACMS missiles against a facility in the Bryansk region close to the border.

"At 03:25 a.m. (0025 GMT), the enemy struck a site in the Bryansk region with six ballistic missiles. According to confirmed data, U.S.-made ATACMS tactical missiles were used," said a Defense Ministry statement.

Lavrov said the 300-kilometer (186-mile) range missiles could not have been fired without U.S. technical assistance.

Moscow has said the use of Western weapons against its internationally recognized territory would make the U.S. a direct participant in the conflict.

Confirmation of the strike came shortly after Putin signed a decree that enables Moscow to use nuclear weapons against non-nuclear states such as Ukraine if nuclear powers support them.

The new doctrine also allows Moscow to unleash a nuclear response in the event of a "massive" air attack, even if it is only with conventional weapons.

Kremlin spokesperson Dmitry Peskov said this was "necessary to bring our principles in line with the current situation."

He also accused the West of trying to use Ukraine to inflict a strategic defeat on Russia, RIA Novosti news agency reported.

'Emboldened Russia'

The 1,000th day of Russia's invasion – launched on Feb. 24, 2022 – comes at a perilous time for Ukrainian forces across the front, particularly near the war-battered cities of Kupiansk and Pokrovsk.

Russia has also intensified strikes on Ukrainian cities in recent days, with attacks on city centers and residential buildings that have killed dozens of civilians.

A Russian strike in the eastern Ukrainian region of Sumy late Monday gutted a Soviet-era residential building and killed at least 12 people, including a child, according to officials.

Ukrainian forces have steadily lost ground in Russia's Kursk region where they seized territory in August and have warned that Russia has massed some 50,000 troops, including North Korean forces, to wrest back the region.

German Chancellor Olaf Scholz told Chinese President Xi Jinping Tuesday that the alleged deployment of North Korean soldiers risked worsening the conflict.

Both Russia and Ukraine have steered their economies to help the war effort.

Ukrainian lawmakers voted Tuesday to approve the 2025 budget with more than $50 billion – or 60% of all expenditure – allocated to defense and security.

Russia's parliament last month approved a budget that will see a defense spending surge of almost 30% next year.

NATO chief Mark Rutte warned Tuesday that Putin must not be allowed to prevail.

"Why is this so crucial that Putin will not get his way? Because you will have an emboldened Russia on our border ... and I'm absolutely convinced it will not stop there," Rutte told reporters in Brussels.

At the United Nations, around 50 member states reaffirmed their support for Kyiv and demanded Moscow withdraw its troops from Ukraine on Tuesday, as they marked the anniversary of Russia's invasion.