North and South Korea have exchanged fire across their land border, South Korean officials say.
The incident came as South Korean activists launched balloons containing leaflets condemning North Korean leader Kim Jong-un.
Pyongyang had warned of "catastrophic" consequences if Seoul allowed the activists to go ahead.
The two sides exchange periodic fire across their disputed western maritime border but incidents on land are rare.
South Korea's Yonhap news agency, citing military officials, said the North fired towards the balloons and South Korea responded after some shots landed south of the border.
There was no immediate information on whether there were any casualties.
Initial reports spoke of artillery fire but Reuters news agency, citing Defence Ministry officials, said that it was gunfire that was exchanged.
The two Koreas are separated by the heavily-fortified Demilitarised Zone. They have technically been at war since the 1950-53 Korean War ended in an armistice rather than a peace treaty.
Groups of activists launch leaflets across the border from time to time, something that always angers North Korea.
This clash came as North Korea marked the 69th anniversary of its ruling Workers' Party.
Observers had been watching the event to see if leader Kim Jong-un attended.
He has not been seen in public for more than a month and there is speculation over both his health and his hold on power.
North Korean media say the young leader, who took over after his father Kim Jong-il died in 2011, is suffering from an "uncomfortable physical condition", without specifying what.
Earlier in the day, South Korea said it believed Mr Kim remained in control of the communist state.