Putin told Xi he was open to discussing a 12-point position paper put forward by Beijing on the conflict, which includes a call for dialogue and respect for all countries' territorial sovereignty.

"It is for Ukraine to decide what are acceptable conditions for any peaceful solution," Stoltenberg said at a press conference, pointing out that China had failed to condemn Moscow's invasion.

"China therefore needs to start to understand Ukraine's perspective and engage with President (Volodymyr) Zelensky directly if it wants to be serious about peace."

The head of the Western military alliance said he welcomed any proposals that could lead to a "just and sustainable peace".

He said China's paper included some positive elements, including emphasising nuclear safety, protecting civilians and territorial integrity.

But he warned any ceasefire that does not respect Ukraine's sovereignty "will only be a way to freeze the war and to ensure that Russia can reconstitute, regroup and reattack".

China has sought to portray itself as a neutral party in the Ukraine conflict, but Washington has said Beijing's moves could be a "stalling tactic" to help Moscow.

Stoltenberg said he had not seen any proof that China is delivering weapons to Russia yet.

"But we have seen some signs that this has been a request from Russia and that this is an issue that is being considered in Beijing," he said.

"Our message has been that China should not provide lethal aid to Russia".