North Korean troops fired multiple gunshots towards the South in the Demilitarized Zone dividing the peninsula on Sunday, prompting South Korean forces to fire back, Seoul said.
The rare exchange of gunfire comes a day after North Korean state media reported that leader Kim Jong Un had made his first public appearance in nearly three weeks following an absence that triggered intense speculation about his health and fears about the stability of the isolated nation.
A South Korean guard post was hit by several shots from the North, the joint chiefs of staff (JCS) in Seoul said in a statement, adding no casualties were reported in the South.
"Our military responded with two rounds of gunfire and a warning announcement," the JCS said.
The South Korean military later said the North Korean gunshots were "not deemed intentional", according to the Yonhap news agency.
The two neighbours remain technically at war after the Korean War ended with an armistice in 1953, and despite its name, the Demilitarized Zone is one of the most fortified places on earth, replete with minefields and barbed-wire fences.
Easing military tensions on their border was one of the agreements reached between Kim and South Korean President Moon Jae-in at a summit in Pyongyang in September 2018.
But most of the deals have not been acted on by North Korea, with Pyongyang largely cutting off contact with Seoul.
North Korea's discussions with the United States over Pyongyang's nuclear arsenal are also at a standstill, despite three meetings between Kim and US President Donald Trump.
The uncertainty around the process would have increased had Kim been incapacitated or dead as rumoured in recent weeks.
North Korean state television, however, showed Kim walking, smiling broadly and smoking a cigarette at what it said was the opening of a fertiliser factory on Friday.
Trump said Saturday he was pleased about Kim's reappearance and that the North Korean leader is apparently healthy.
"I, for one, am glad to see he is back, and well!" the president tweeted.
Before Kim's reappearance, US Secretary of State Mike Pompeo said last month that he remained hopeful a nuclear deal could be clinched with North Korea.
"Regardless of what transpires inside of North Korea with respect to their leadership, our mission remains the same -- to deliver on that commitment that Chairman Kim made with President Trump... (the) verified denuclearisation of North Korea," Pompeo told reporters.
"We are still hopeful that we'll find a path to negotiate that solution to get the outcome that is good for the American people, good for the North Korean people and for the whole world."