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 since Russia invaded Ukraine, Foreign Minister Sergei Lavrov criticised Washington's decision to authorise Kyiv to use such missiles, which have a range of up to 300 kilometres (186 miles).

"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.

The grim milestone opened with a Russian strike in the eastern Ukrainian region of Sumy that gutted a Soviet-era residential building and killed at least 12 people, including a child.

President Volodymyr Zelensky published images of rescue workers hauling bodies from the debris and called on Kyiv's allies to "force" the Kremlin into peace.

The foreign ministry said Ukraine "will never submit to the occupiers" and called for "peace through strength, not appeasement," referring to growing calls for Ukraine to negotiate with Russia to end the war.

US president-elect Donald Trump has vowed to cut US assistance to Ukraine and bring about a swift end to the war, without detailing how he would do so.

- 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 missiles could not have been fired without US technical assistance and said the strikes showed the West and Kyiv want "escalation".

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

The strike confirmation 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.

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 meanwhile 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.

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.