By Ju-min Park and Tony Munroe
SEOUL (Reuters) - South Korea fired tens of artillery rounds toward North Korea on Thursday after the North launched shells to protest South Korea's anti-Pyongyang propaganda broadcasts along the border, as tension escalated on the peninsula.
North Korea did not 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 defense 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 Aug. 10, would continue.
South Korea said the North fired a 14.5 mm anti-aircraft shell at 3:52 p.m. (0652 GMT), then fired multiple shells from a 76.2 mm direct fire weapon at 4:15 p.m.
No damage or injuries were reported in the South.
South Korean President Park Geun-hye told top defense officials to "react firmly" to North Korean provocations, a spokesman quoted her as saying.
South Korea's military, which said it fired 155 mm artillery rounds in response, raised its alert status to the highest level.
"Our military has stepped up monitoring and is closely watching North Korean military movements," South Korea's defense ministry said.
There was no mention of the firing in isolated North Korea's state media, which does not typically make immediate comment on events.
The first North Korean shell landed in an area about 60 km (35 miles) north of Seoul in the western part of the border zone, the defense ministry said. Nearly 800 South Korean residents living close to the border were ordered to evacuate and stay in shelters, according to officials from Gyeonggi province and the city of Incheon.
The exchange of fire was the first between the two Koreas since last October, when North Korean soldiers approached the military border and did not retreat after the South fired warning shots, the South Korean Defense Ministry said at the time. The North's soldiers fired back in an exchange of gunfire that lasted about 10 minutes, with no casualties.
Tension between the two Koreas has risen since early this month when landmine explosions in the Demilitarized Zone (DMZ) of the border wounded two South Korean soldiers. Seoul accused North Korea of laying the mines, which Pyongyang has denied.
Seoul then began blasting anti-North Korean propaganda from loudspeakers on the border, resuming a tactic that both sides had halted in 2004.
North Korea on Saturday demanded that the South stop the broadcasts or face military action, and on Monday began conducting its own broadcasts.
Thursday's exchange of fire came amid ongoing annual joint U.S. and South Korean military exercises, which began on Monday and which North Korea condemns as preparation for war.
The two Koreas have remained in a technical state of war since the 1950-1953 Korean War ended in a truce, not a peace treaty.
South Korea's won currency weakened in non-deliverable forward trading on the reports of the firing, which came after onshore spot trading had closed. The 1-month contract <KRW1MNDFOR=> rose as high as 1,192.7 won per dollar from around 1,189.8 earlier.
(Additional reporting by James Pearson, Christine Kim and Choonsik Yoo; Editing by Raju Gopalakrishnan and Ryan Woo)