North and South Korea have exchanged gunfire in the Demilitarized Zone (DMZ) which divides the two countries.
Seoul's military said shots from the North hit a South Korean guard post in the central border town of Cheorwon.
It said it returned fire and delivered a warning announcement. There are no reports of injuries.
Such incidents across the world's most heavily fortified border are rare - the last occurred in 2017. It is not clear what led the North to open fire.
US Secretary of State Mike Pompeo has said the shots fired from North Korea were believed to have been "accidental".
Military officials in the South say there was no sign of unusual troop movements. They are trying to find out if this was an accident or deliberate.