After Russia sent troops into Ukraine, China sought to position itself as neutral but at the same time deepened ties with Moscow, and has not condemned the invasion.

Speaking on the sidelines of the Munich Security Conference, Stoltenberg said NATO was "following closely the increased and stronger relationship between China and Russia".

The countries were conducting military exercises together, as well as naval and air patrols, he said.

"When authoritarian powers are coming closer and working more closely together, it's even more important that all of us that believe in democracy and freedom stand together in NATO, and with our partners throughout the world," he said.

Stoltenberg said that Beijing was watching the war in Ukraine closely.

"If President (Vladimir) Putin wins there, it will impact the calculations and decisions they will make in Beijing," he said.

Russia's war in Ukraine has stoked fears among Western powers that China could try something similar in Taiwan, a self-ruled democratic island that Beijing claims as part of its territory.

The Munich Security Conference, which began Friday, is being attended by world leaders including German Chancellor Olaf Scholz, French President Emmanuel Macron, US Vice President Kamala Harris and China's top diplomat Wang Yi.