Stoltenberg is in Seoul, the first stop on a trip that will include Japan and is aimed at strengthening ties with US allies in the face of the war in Ukraine and rising competition with China.
Speaking at the Chey Institute for Advanced Studies in Seoul on Monday, he thanked South Korea for its non-lethal aid to Ukraine but urged it to do more, adding there was an "urgent need" for ammunition.
"I urge the Republic of Korea to continue and to step up on the specific issue of military support," he said.Â
In meetings with senior South Korean officials, Stoltenberg argued that events in Europe and North America were interconnected with other regions, and the alliance wanted to help manage global threats by increasing partnerships in Asia.
South Korea has signed major deals providing hundreds of tanks, aircraft and other weapons to NATO member Poland since the war began, but South Korean President Yoon Suk-yeol has said his country's law against providing arms to countries in conflicts makes providing weapons to Ukraine difficult.
Stoltenberg noted that countries such as Germany, Sweden, and Norway had similar policies but changed them.
"If we don't want autocracy and tyranny to win, then they need weapons, that's the reality," he said, referring to Ukraine.
The NATO chief said it was "extremely important" that Russia didn't win this war, not only for the Ukrainians but also to avoid sending a wrong message to authoritarian leaders, including in Beijing, that they could get what they wanted by force.
Although China was not NATO's adversary, it had become "much higher" on NATO's agenda, Stoltenberg said, citing Beijing's rising military capabilities and coercive behaviour in the region.
Last year South Korea opened its first diplomatic mission to NATO, vowing to deepen co-operation on non-proliferation, cyber defence and other security areas.
The NATO chief's visit also comes as US Defence Secretary Lloyd Austin was due to arrive for talks in Seoul on Monday.
