South Korea says it has begun shelling North Korean military positions. The attack follows a North Korea shell that allegedly landed in a South Korean border town.
South Korea's defense ministry said Thursday that its rival North Korea had launched a shell at a border town and it had retaliated.
"Our side staged a counter-attack with dozens of 155mm shells," a South Korea ministry spokesman told the AFP news agency.
The suspected North Korean projectile landed in an area about 60 km (35 miles) north of Seoul on the western part of the border zone, the defense ministry said. South Korean residents in the area were ordered to evacuate, according to the South Korea's Yonhap News Agency.
The reports said South Korea's presidential office had convened its national security council.
North Korea had warned it would attack loudspeakers in South Korean broadcasting anti-North propaganda.
Seoul had recently started the broadcasts recently in response to what it said were North Korean-laid land mines that maimed two of its soldiers.
"Our military has stepped up monitoring and is closely watching North Korean military movements," South Korea's defense ministry said in a statement.
The cross border shelling comes at a time of rising tensions as the US and South Korea are conducting joint military exercises this week.
North Korea considers the maneuvers a rehearsal for an invasion and had previously warned of attacks if the exercises continued.