President Trump welcomed the news of Kim Jong-un's public appearance at an opening ceremony of a new plant, following weeks of rumors about the North Korean leader's health, fueled in part by Trump's own vague statements.
Seoul's military said shots from the North hit a guard post in the central border town of Cheorwan, located in the Demilitarized Buffer Zone (DMZ). Over the years, the two militaries have occasionally exchanged gunfire.
The DMZ, which is about 155 miles long and 2.5 miles wide, has often been a flashpoint in tensions between the rival nations, which remain technically at war since their 1950-53 conflict ended in an armistice instead of a peace treaty.
Despite its name, the demilitarised zone is one of the most fortified places on Earth, replete with minefields and barbed-wire fences.
The South responded with a total of 20 rounds of warning shots on two occasions before issuing a warning broadcast, it said.
North Korea's official media published photographs and a report on Saturday that Kim had attended the completion of a fertiliser plant, the first report of his appearance since April 11.
The South Korean authorities made it clear that the incident is in violation of the bilateral military accord of the Comprehensive Military Agreement signed in September 2018, and urged the North to fully abide by the agreement.
But most of the deals have not been acted on by North Korea, with Pyongyang largely cutting off contact with Seoul.
"Our mission remains the same, to convince the North Koreans to give up their nuclear weapons and create a brighter future for the North Korean people".
The uncertainty around the process would have increased had Kim been incapacitated or dead as rumored in recent weeks.
Speculation about Kim's health has been swirling since his conspicuous no-show at April 15 celebrations for the birthday of his grandfather, the North's founder - the most important day in the country's political calendar. He reappeared in mid-October, with South Korea's intelligence agency saying that he had probably had an operation on his left ankle as a result of problems with a cyst.
In images of the factory visit, Kim was seen accompanied by his younger sister Kim Yo Jong, who has been talked about by experts as a possible successor, although no woman has ever ruled North Korea.
Mr Kim's absence caused intense speculation about his health and fears about the stability of North Korea.
The Government official declined to provide reasons, but said speculative reports that Mr Kim had had an operation, citing some differences in his leg movements, were untrue.