Yoon made the remark in a written interview with Spanish news agency EFE ahead of a trip to South America for the Asia-Pacific Economic Cooperation (APEC) summit in Peru and the Group of 20 (G20) summit in Brazil.
Both Seoul and Washington have confirmed that North Korean soldiers, who have been deployed to Russia's western front-line Kursk region, have begun engaging in combat against Ukrainian forces, reports Yonhap, quoting the interview.
"If they both do not stop their dangerous military adventure, we are prepared to implement appropriate effective measures, including strengthening support for Ukraine, in cooperation with our allies and like-minded countries," Yoon said in the interview.
Yoon cautioned against Moscow transferring sensitive military technology to Pyongyang in exchange for North Korean troop deployment, expressing concerns over North Korean forces gaining modern warfare experience in Ukraine.
He called on Pyongyang and Moscow to end their "illegal" military cooperation and withdraw North Korean troops from Russian territory while noting South Korea is maintaining diplomatic channels with Russia.
Yoon said Seoul is also committed to maintaining a "strategic dialogue" with China, an ally of both North Korea and Russia, underscoring China's potential role in promoting regional stability.
At the upcoming APEC and G20 summits, Yoon said he will advocate free, open world trade, emphasizing Latin America's growing "strategic importance." It marks Yoon's first official trip to Latin America since taking office in May 2022.