Defence ministry said on Thursday that South Korean military detected a projectile from the North, presumed to be a rocket, into the South's Yeonchon area.
The ministry also said that the South Korean military fired "tens" of 155mm artillery rounds to the origin point of the North Korean projectile after the incident.
The suspected North Korean projectile landed in an area about 60km north of Seoul in the western part of the border zone, the defence ministry said.
"Our military has stepped up monitoring and is closely watching North Korean military movements," the defence ministry said.
North Korea demand
The South Korean defence ministry said that North Korea demanded the South to stop the propaganda broadcasting within 48 hours and dismantle all equipment, else it would initiate military actions.
The official Yonhap news agency said that Seoul will hold a National Security Council session over the shelling.
South Korean residents in the area were ordered to evacuate, according to the agency.
On August 4, two South Korean soldiers were seriously injured in a landmine explosion in the southern part of the demilitarised zone, causing them to lose limbs.
