NATO has "seen some signs" that Russia has likely requested lethal aid from China to bolster Moscow’s war in Ukraine, the alliance’s secretary general said Tuesday.

“We haven’t seen any proof that China is delivering lethal weapons to Russia, but we have seen some signs that this has been a request from Russia, and that this is an issue that is considered in Beijing by the Chinese authorities,” Jens Stoltenberg told reporters at a news conference in Brussels.“China should not provide lethal aid to Russia. That would be to support an illegal war,” Stoltenberg warned.

The NATO chief's comments come as China's Xi Jinping meets with Russian President Vladimir Putin in Moscow. Since Russia began its invasion of Ukraine, Beijing has claimed impartiality in the conflict, but echoed Kremlin rhetoric blaming NATO for the conflict, refused to condemn the invasion, and backed Moscow financially by significantly increasing purchases of Russian fuel.

Stoltenberg said Xi's three-day state visit to Moscow signals the closer ties developing between China and Russia in recent years.

“We see how China and Russia are coming closer and closer in the military domain — joint exercises, joint patrols, naval and air patrols — in the economic domain and also in the political and diplomatic domain,” Stoltenberg said. “So the meeting in Moscow is part of that pattern where China and Russia are working more and more closely and building a stronger and stronger partnership.”