The North Korean soldier bolted from a guard post at the northern side of Panmunjom to the southern side of the village, South Korea's Joint Chiefs of Staff said in a statement.
Three carriers will be likely together around Monday, according to a South Korean military source who did not like to be named.
Also on Saturday, the US and South Korean navies launched four days of joint drills off the east coast of South Korea.
The country's joint chiefs of staff (JCS) announced Friday that South Korea will send seven of its warships-including two Aegis Combat System-equipped vessels, one destroyer and four frigates-to join three USA aircraft carriers deployed in the Western Pacific that will perform their first joint drill in a decade.
The aircraft carrier USS Ronald Reagan conducts joint drills with the South Korean navy at sea on October 19.
It is the first such triple-carrier drill in the region for a decade.
In recent months, North Korea has tested intercontinental ballistic missiles that could reach the United States mainland with further development and has conducted its most powerful nuclear test.
He was wounded from North Korean firing and was being taken to a hospital, it said.
The exercises come on the heels of Trump's visits to Seoul and Beijing this week, which were dominated by the question of how to counter Pyongyang's nuclear weapons threat.
He was shot by a North Korean soldier but succeeded in crossing, the JCS added, and has been evacuated for medical treatment.
During his visit to South Korea during his ongoing Asian tour, US President Donald Trump strongly criticised Pyongyang and twice referred to the deployment of the three aircraft carriers in the region.
Xi repeated his plea for the issue to be resolved through negotiations, saying China was ready to discuss the "pathway leading to enduring peace and stability on the peninsula".
But experts doubt China will take the kind of steps that Trump wants, such as halting crude oil exports to the North.