Beijing: North and South Korea have exchanged fire along the countries' western border, in the latest escalation of cross-border violence.
North Korea fired at a South Korean military loudspeaker that had been blaring anti-Pyongyang broadcasts in recent days, prompting South Korea to respond by launching "dozens of shells" at the source of the attack, South Korean media reported.
There were no immediate reports of casualties or damage on either side.
State-runKBS News quoted a South Korean military official as saying that the North Korean military opened fire at 3.52pm local time on Thursday.
"A trajectory of supposedly North Korean military shells has been detected by [South Korea's radar] detection system," a defence ministry official said.
North Korea did not immediately respond to the South's return fire but later warned Seoul in a letter that it would take military action if the South did not stop the loudspeaker broadcasts within 48 hours, the South's defence ministry said.
In a separate letter, Pyongyang said it was willing to offer an opening to resolve the conflict even though it considers the broadcasts a declaration of war, South Korea's Unification Ministry said.
A South Korean military official said the broadcasts, which began on August 10, would continue as it raised its alert status to the highest level.
The South was to hold an urgent national security council session over the attack and ordered a civilian evacuation near the western border, Yonhap reported.
South Korean President Park Geun-hye told top defence officials to "react firmly" to North Korean provocations, a spokesman quoted her as saying.
"Our military has stepped up monitoring and is closely watching North Korean military movements," South Korea's defence ministry said.
Relations between North and South have been strained after Seoul accused Pyongyang of planting landmines that exploded earlier this month, wounding two South Korean soldiers.
In retaliation, Seoul restarted cross-border propaganda broadcasts on loudspeakers that had sat dormant for nearly a decade.
North Korea denied planting the landmines and last week threatened "all-out military action" against "anti-North Korean psychological warfare" if the South Korea did not immediately cease the broadcasts.
Tensions routinely flare up between the two Koreas, separated by a heavily fortified de-militarised zone. The two sides are technically still at war after their 1950s conflict ended in truce rather than a peace treaty.
Thursday's exchange of fire came amid ongoing annual joint US and South Korean military exercises, which began on Monday and which North Korea regards as preparation for war.