"But our military is keeping a full readiness posture against the possibility of North Korea's provocations", a military official said in a briefing.
South Korea's National Intelligence Service (NIS) confirmed a U.S. citizen had attempted to enter North Korea via the DMZ, but would not give any further information.
In a statement, the Joint Chiefs of Staff (JCS) said: "The US Navy mobilised three aircraft carriers and 11 Aegis destroyers, while the Republic of Korea Navy mobilised seven warships including two Aegis destroyers".
The defector, dressed in a combat uniform, was unarmed, Al Jazeera reported.
How many North Koreans defect annually?
File image of a North Korean soldier, centre top, looks at the southern side as South Korean soldiers stand guard at the border village of Panmunjom.
He was shot in the shoulder and elbow and was taken to a South Korean hospital, the Defence Ministry said.
From January to August 2017, 780 North Koreans escaped to South Korea, according to officials in Seoul.
Panmunjomis where an armistice was signed to pause the Korean War.
The hermit kingdom official then blasted the UN for "turning a blind eye to the nuclear war exercises of the United States, who is hell bent on bringing a catastrophic disaster to humanity". Trump's lengthy trip centered on trade and the North Korean missile and nuclear crisis.
The most famous incident was in 1976, when two American army officers were killed by ax-wielding North Korean soldiers.
North Korea conducted its sixth and most powerful nuclear bomb test on September 3, but has not launched any missiles since firing one over Japan on September 15, the longest such lull this year.
About 1,000 people from the North flee to the South each year - but very few defect via the DMZ.
The Associated Press contribute to this report.