During the summit, Yoon said he would seek "concrete results" from the efforts of the Allies to improve their response to emerging threats from North Korea, which has stepped up military testing and launched its first solid-fuel intercontinental ballistic missile last week.
Yoon said in an interview with Reuters ahead of his official visit to the United States next week, "Seoul will, on its part, increase its surveillance, reconnaissance and intelligence analysis capabilities, and develop ultra-high-performance, high-powered weapons to fend off the North's threats."
"If a nuclear war breaks out between South and North Korea, it probably won't just be a problem between the two sides, but probably all of Northeast Asia will be turned to ashes. This has to be prevented," he said.
Yoon also signaled for the first time a change in his stance against Ukraine's arming.
South Korea has announced that it can expand its support for Ukraine beyond humanitarian and economic aid if it is subjected to a large-scale civilian attack.
Yoon said his government is investigating how it can help defend and rebuild Ukraine.
Seoul has suggested for the first time in more than a year that it is willing to supply arms to Ukraine, after ruling out the possibility of "deadly weapons assistance."
“It may be difficult for us to insist on humanitarian or financial support alone if there is a situation that the international community cannot condone, such as a large-scale attack on civilians, a massacre, or a serious violation of the laws of war,” Yoon said.
South Korea, an important ally of the United States and a major producer of artillery, has so far tried to avoid turning Russia into an enemy due to Moscow's influence on North Korea, despite increasing pressure to procure weapons from Western countries.