South Korea's military said he suffered injuries in his internal organs but wasn't in a life-threatening condition.
The soldier was being operated upon for a second time on Wednesday, the hospital treating him said. Exercises closest to the Korean coast involved USA and South Korean ships, while those closest to Japan involved United States and Japanese ships, the official said.
A spokesman for South Korea's Ministry of National Defense said on Thursday that no joint military drills involving all three countries had been planned.
President Donald Trump is now concluding his tour of Asia, which involved multiple discussions with regional leaders including Japenese Prime Minister Shinzo Abe, Chinese President Xi Jinping, and Filipino President Rodrigo Duterte regarding how to address the North Korean threat.More news: I'm A Celeb bosses reportedly increase security around Ant McPartlin
Ja issued his claim as the USA and South Korean militaries continue to conduct military exercises in the region with both military aircraft and major warships, in what South Korea claimed was an attempt to show Pyongyang any aggression can be repelled with "overwhelming force".
The JSA is a small strip of land on the border between the Koreas where troops from both sides stand face-to-face.
On Tuesday, Abe warned that despite North Korea's failure to test a weapon since September, there was no sign the regime was scaling back its military plans and therefore talks would be futile.
More than 30,000 North Korean civilians have fled their homeland since the two nations came into being in 1948.
Two North Korean soldiers came over to the South in June after crossing the frontier.More news: Russia's State Duma passes amendments to foreign agent media law
The JSA is jointly overseen by the American-led U.N. Command and by North Korea, with South Korean and North Korean border guards facing each other only meters (feet) apart.
The JCS said that the South Korean Army has increased its alert level in anticipation of possible North Korean provocations. It is located inside the 4-kilometer-wide (2 1/2-mile-wide) Demilitarized Zone, which forms the de facto border between the Koreas since the Korean War.
"Since it was an area exposed to the North, we had to crawl toward there to get him out", the official added.
The Joint Security Area was the site of some bloodshed during the Cold War but there hasn't been major violence there in recent years.
Yun's visit closely follows US President Donald Trump's state visit here last week, where he addressed North Korea's nuclear ambitions and its threats to regional security.More news: Election Commission bars BJP from using the term 'Pappu' in Gujarat elections
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