Russia warned on Tuesday that it would respond after Ukraine fired longer-range US missiles at its territory for the first time, as President Vladimir Putin issued a nuclear threat on the 1,000th day of the war.
A senior official told AFP that a strike on Russia's Bryansk region earlier on Tuesday "was carried out by ATACMS missiles" -- a reference to the US-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 G20 summit in Brazil.
Putin signed a decree on Tuesday lowering the threshold for using nuclear weapons, a move that the White House, UK 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 US.
Ukrainian leader Volodymyr Zelensky accused G20 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 cuts military funding to Kyiv.
US 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," Zelensky said in an interview with US network Fox News.
"We will fight. We have our production, but it's not enough to prevail," he added.
Washington this week said it had cleared Ukraine to use ATACMS against military targets inside Russia -- a long-standing Ukrainian request.
The Washington Post on Tuesday -- citing two unnamed officials -- reported Biden had also authorised the provision of antipersonnel land mines to Ukraine, a step that will bolster Kyiv's defences against advancing Russian troops but has drawn criticism from arms control groups.
Russia said on Tuesday that Ukraine had used the ATACMS missiles against a facility in the Bryansk region close to the border.
"At 03:25 am (0425 GST), the enemy struck a site in the Bryansk region with six ballistic missiles. According to confirmed data, US-made ATACMS tactical missiles were used," said a defence ministry statement.
Lavrov said the 300-kilometre (186-mile) range missiles could not have been fired without US technical assistance.
Moscow has said the use of Western weapons against its internationally recognised territory would make the US 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 they are supported by nuclear powers.
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 spokesman 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.
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