"It is President (Vladimir) Putin who started this imperial war of conquest. It is Putin who keeps escalating the war," Stoltenberg said.

"We are also increasingly concerned that China may be planning to provide lethal support for Russia's war."

Stoltenberg was speaking after a meeting with EU foreign policy chief Josep Borrell and Ukraine's Foreign Minister Dmytro Kuleba on plans to step up Western ammunition supplies to Ukraine.

The Western allies worry they are falling behind in supplying enough shells for Kyiv's artillery to fend off a renewed Russian offensive.

But if the fears -- first raised by Washington -- that China is preparing to deliver weapons to Russia are realised, they could fall even further behind in what would be a growing arms supply race.

Borrell said he had raised the issue with his Chinese counterpart Wang Yi and received assurances that China would not supply weapons to any country at war.

"We have to remain vigilant, but as far as I know, there is no evidence that China has been doing what they claim not to be doing," Borrell said.