Moscow has reacted furiously to a decision by US President Joe Biden to change policy on Ukraine and allow Kyiv to use US-supplied long-range missiles to strike Russian territory for the first time.

On Tuesday, 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.

Following the decree, 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.

On Wednesday Beijing called for a cooling of tensions.

"Under the current circumstances, all parties should remain calm and exercise restraint, working together through dialogue and consultation to ease tensions and reduce strategic risks," foreign ministry spokesman Lin Jian said when asked about the decree and the strikes inside Russia.

"China's stance of encouraging all parties to de-escalate the situation and commit to a political resolution of the Ukraine crisis remains unchanged," Lin added.

"China will continue to play a constructive role in this regard," he said.

The White House, UK and European Union condemned Putin's nuclear decree as "irresponsible".

Speaking to journalists after the G20 summit in Rio de Janeiro, French leader Emmanuel Macron said he had asked Chinese President Xi Jinping during a meeting at the G20 to "use all his influence" with Putin to try to achieve a de-escalation.

Macron said Russia ally China had "the capacity to negotiate with President Putin so that he halts his attacks" on Ukraine.

China presents itself as a neutral party in the Ukraine war and says it is not sending lethal assistance to either side, unlike the United States and other Western nations.

But it remains a close political and economic ally of Russia and NATO members have branded Beijing a "decisive enabler" of the war, which it has never condemned.