South Korea fired a barrage of artillery rounds into North Korea on Thursday after the North shelled across the border to protest against anti-Pyongyang propaganda broadcasts by Seoul, moves that raised tensions on the divided peninsula.
Washington urged Pyongyang to halt any "provocative" actions in the wake of the first exchange of fire between the two Koreas since last October. Both sides said there were no casualties or damage in their territory.
North Korea did not return fire but warned Seoul in a letter that it would take military action if the South did not stop the broadcasts along the border within 48 hours, the South's Defense Ministry said.
In a separate letter, Pyongyang said it was willing to resolve the issue even though it considered the broadcasts a declaration of war, South Korea's Unification Ministry said.
North Korea's young leader, Kim Jong Un, would put his troops on a "fully armed state of war" starting from 5 p.m. on Friday and had declared a "quasi-state of war" in frontline areas, Pyongyang's official KCNA news agency reported.
Such language is often used by North Korea in times of tension with the South.
A South Korean military official said the broadcasts would continue. Seoul began blasting anti-North Korean propaganda from loudspeakers on the border on Aug. 10, resuming a tactic that both sides had stopped in 2004.
South Korea said the North had fired one anti-aircraft shell followed by multiple shells on Thursday.
South Korea's military, which said it fired "tens" of artillery rounds in response, raised its alert status to the highest level.
South Korean President Park Geun-hye told defense 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 Defense Ministry said.
The North Korean army said the South fired 36 rounds, six of which landed near its guard posts, in a "reckless provocation," KCNA said.
The United States, which has about 28,500 military personnel in South Korea, said it was concerned and closely monitoring the situation.
"Such provocative actions heighten tensions, and we call on Pyongyang to refrain from actions and rhetoric that threaten regional peace and security," U.S. State Department spokesperson Katina Adams said.
The Pentagon said it would "take prudent measures" to ensure the well-being of U.S. personnel, but did not elaborate.
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 nearby were ordered to evacuate and stay in shelters, officials said.
North Korea said the South's military "invented a case of 'shell fired by the North'," according to KCNA.
The two Koreas last exchanged fire in 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. There were 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.
The incident prompted Seoul to stage the propaganda broadcasts.
North Korea on Monday began conducting its own broadcasts.
Thursday's exchange of fire took place during annual joint U.S. and South Korean military exercises.
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.