Beijing, China - China called for Russia to exercise "calm" and "restraint" after President Putin threatened to use nuclear weapons against Ukraine.

Moscow reacted furiously to US President Joe Biden's decision to allow Kyiv to use US-supplied long-range missiles to strike inside Russian territory for the first time since the invasion of Ukraine.

The move led to a ramp-up of Russia's nuclear rhetoric after Ukraine launched six missiles into the Russian region of Bryansk on Tuesday.

In response to the strikes, Russian Foreign Minister Sergei Lavrov said that they had entered "a qualitatively new phase" of what he termed the "Western war against Russia."

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Russian President Vladimir Putin also signed a decree allowing Moscow to use nuclear weapons against non-nuclear states who are themselves supported by nuclear powers, paving the way for a potentially catastrophic escalation in Ukraine.

Beijing weighed in on Wednesday, urging "calm" and "restraint" during foreign ministry spokesperson Lin Jian's regular press conference. China also vowed to play a "constructive role" in ending the conflict.

"Under the current situation, all relevant parties need to remain calm and restrained and jointly seek deescalation and lower strategic risks through dialogue and consultation," spokesperson Lin said.

"Let me stress that China calls for all parties to deescalate the situation and strive for the political settlement of the Ukraine crisis. This position remains unchanged. China will continue to play a constructive role to this end."