Multiple shots were fired by North Korea towards the South in the Demilitarised Zone, South Korea says.
North and South Korea have exchanged gunfire in the Demilitarised Zone which divides the two countries.
Gunshots fired by North Korea at 07:41 (23:41 BST) hit a South Korean guard post in the central border town of Cheorwon, Seoul's military said.
No casualties were reported on the South Korean side.
In response, South Korea fired "two rounds of gunfire and a warning announcement according to our manual", the military statement said.
It added that officials were trying to contact North Korea.
It is not clear what provoked the initial gunshots.
This is the first time in five years that North Korean troops have directly fired on the South.
The demilitarised zone (DMZ) was set up after the Korean War in 1953 in order to create a buffer zone between the two countries.
For the past two years, the government in Seoul has tried to turn the heavily fortified border into a peace zone.
This increase in tensions comes a day after North Korean leader Kim Jong-un appeared in public, following an almost-three-week unexplained absence that sparked intense global speculation about his health. BBC