NATO's secretary general has expressed caution over the latest summit between Russian President Vladimir Putin and Chinese President Xi Jinping in Moscow.

Jens Stoltenberg told reporters in Brussels on Tuesday that their meeting is a part of that pattern where China and Russia are working more closely and building a stronger partnership.

Asked about the possibility of Beijing providing military aid to Moscow, Stoltenberg said, "We haven't seen any proof that China is delivering lethal weapons to Russia."

But he added that NATO has seen some signs that Russia has requested an arms delivery from China and that the issue is being considered by the Chinese authorities.

The NATO chief said, "Our message has been that China should not provide lethal aid to Russia. That will be to support an illegal war."