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.

Russian President Vladimir 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".

Putin 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 at a summit in Brazil of failing to act over Putin's nuclear threats, saying the Russian leader had no interest in peace.

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.

- Nuclear sabre-rattling -

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

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

"At 03:25 am (0025 GMT), 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 Russian President Vladimir 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 nuclear 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."

- 'Direct threat' to West -

The 1,000th day of Russia's invasion -- launched on February 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 centres and residential buildings that have killed dozens of civilians.

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 by Russia in its war against Ukraine risked worsening the conflict.

Both sides 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 percent of all expenditure -- allocated to defence and security.

Russia's parliament last month approved a budget that will see a defence spending surge of almost 30 percent 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.

"It is then posing a direct threat to all of us in the West," he said.

The EU's outgoing top diplomat Josep Borrell also pressed member states to align with Washington in allowing Kyiv to strike inside Russia using donated long-range missiles.

"It is fully in accordance with international law," he said.