Kyiv, Ukraine - Ukraine said Tuesday it had fired US-supplied long-range missiles into Russian territory in the first such strike, as Moscow declared a "new phase" in the conflict and eased its protocol for nuclear strikes.

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 criticized the decision to authorize the use of such missiles, which have a range of up to 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.

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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.

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.

The Biden administration this week 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, 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 defense ministry statement.

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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 vies the use of Western weapons against its internationally recognized territory as confirmation that 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 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."