South Korea stands ready to respond to further provocations from North Korea, the presidential Blue House said on Saturday, as an ultimatum loomed for Seoul to halt anti-Pyongyang propaganda broadcasts by late afternoon or face military action.
Tension on the Korean peninsula has been running high after an exchange of artillery fire on Thursday, prompting calls for calm from the United Nations, the United States and the North's lone major ally, China.
North Korea, technically still at war with the South after their 1950-53 civil conflict ended in a truce, not a treaty, has declared a "quasi-state of war" in front-line areas and set a deadline of 5pm Pyongyang time (0830 GMT) for Seoul to halt the broadcasts from loudspeakers along the border.
Seoul says it will continue the broadcasts unless the North accepts responsibility for landmine explosions this month in the Demilitarised Zone (DMZ) that wounded two South Korean soldiers. Pyongyang denies it planted the mines.
South Korean vice-defence minister Baek Seung-joo said on Friday his government expected North Korea to fire at some of the 11 sites where Seoul has set up loudspeakers.
A military source told South Korea's Yonhap news agency on Saturday that there were signs the North Korean military was preparing to attack the loudspeakers, towing artillery to near the border. The defence ministry said it was checking the report.
"The Blue House is calmly observing the situation and ready to respond strongly against any additional provocations," a spokesperson told Reuters.
North and South Korea have often exchanged threats over the years, and dozens of soldiers have been killed in clashes, yet the two sides have always pulled back from all-out war. Analysts expect this crisis eventually to wind down.
But the tension represents a blow to South Korean President Park Geun-hye's efforts to improve North-South ties, which have been virtually frozen since the deadly 2010 sinking of a South Korean warship. North Korea denies it was involved.
"The whole situation on the Korean peninsula has been bad for years now, because there's very little intra-Korean contact and that only makes a bad situation worse," said Joel Wit of 38 North, a North Korea monitoring project at Johns Hopkins University in Washington.
Meanwhile, both nations traded harsh rhetoric late into Friday night.
South Korean government employees were ordered into emergency duty starting Friday, meaning they must work until 11pm, including during the weekend, the ministry of personnel management told Reuters
Near the border, 58 people evacuated from a village on the South Korean side, where a shell landed on Thursday, had returned home although the village remained off-limits to outsiders and DMZ tours had been halted, an official said.
At the jointly-run Kaesong Industrial Complex just north of the border, the last significant vestige of cooperation spawned by the rivals' first summit meeting 15 years ago, entries and exits were proceeding as normal, Yonhap said.
South Korea's unification minister said it was considering bringing home 83 South Koreans who are in Pyongyang for a youth football tournament if the situation worsens, YTN TV reported.
South Korea began blasting anti-North propaganda over the DMZ on August 10, resuming a tactic both sides had stopped in 2004, days after the landmine incident.
North Korea resumed its own broadcasts on Monday and on Thursday, according to Seoul, launched four artillery shells into South Korea in apparent protest at the broadcasts. The South fired back 29 artillery rounds. Pyongyang accused the South of inventing a pretext to fire into the North.
The United States, which has 28,500 military personnel based in South Korea, said on Friday it had resumed its annual joint military exercises there after a temporary halt to coordinate with Seoul over the shelling from North Korea.
The annual exercises, code-named Ulchi Freedom Guardian, began on Monday and run until next Friday.
North Korea regularly condemns the manoeuvres as preparation for war and in the past has fired rockets into the sea while they proceeded.
North Korea has been hit with UN and US sanctions because of its nuclear and missile tests, moves that Pyongyang sees as an attack on its sovereign right to defend itself.
Neither side reported casualties or damage from Thursday's shelling, indicating those exchanges were just warning shots.