SEOUL, South Korea — Gunshots fired from North Korea struck a guard post in South Korea on Sunday inside the Demilitarized Zone separating the two countries, and South Korean soldiers fired back, the South’s military said.

       There were no casualties reported on the South Korean side, the country’s military said in a brief statement.

       The tense, 155-mile border is the most militarized in the world. The South said it was contacting the North through a military hotline to prevent the situation from escalating. It remained unclear why the North fired the shots.

       The exchange of gunfire came a day after North Korea reported the first public appearance by the country’s leader, Kim Jong-un, in three weeks. Mr. Kim’s absence from public view had spawned a torrent of rumors and unconfirmed news reports that he was gravely ill after having heart surgery.

       The border is filled with mines, guard posts, fences and has nearly two million battle-ready troops on both sides. Over the years, the two militaries have occasionally exchanged gunfire.

       The two Koreas remain technically in a state of war after the 1950-53 Korean War was halted in a truce, not in a peace treaty.

       One of the most serious exchanges of gunfire at the border took place in 2010, when North Korea launched an artillery barrage at a South Korean border island, killing two South Korean marines and two civilians. South Korea launched a counter-barrage at the North Korean side.

       Earlier that year, 46 South Korean sailors were killed when their ship sank in waters near the island in what the South called a torpedo attack from a North Korean submarine.