Four live rounds were fired during a training along the border in Gangwon province on Saturday evening, Yonhap news agency reported, citing South Korean military officials.

All of the bullets landed on the South's side and no damage was reported.

The military unit immediately informed North Korea that the firings were not intentional and stepped up readiness posture, the officials said.

"No particular signs have been detected from the North's side, and an investigation is under way over the exact circumstances of the incident," an unnamed military official told Yonhap.

The two Koreas technically remain at war after fighting was halted by an armistice in 1953, and are separated by the four-kilometre-wide demilitarised zone (DMZ) that runs for 250 kilometres (160 miles) across the Korean peninsula.

Despite its name, the DMZ is one of the most fortified places on earth, replete with minefields and barbed-wire fences.

The last time the two sides exchanged fire on the border was in May 2020, when at least four bullets from North Korea hit South Korea's guard post at the central part of the DMZ, prompting Seoul's troops to fire back.

North Korean soldiers also shot at a defector in 2017 but the South did not fire back.