SEOUL, South Korea — North Korea on Tuesday blew up a building where its officials and their South Korean counterparts had recently worked side by side, dramatically signaling its displeasure with the South after weeks of threats to end the countries’ recent détente.
South Korean border guards heard an explosion and then saw smoke rising from Kaesong, the North Korean town where the building was located. The building appeared to be blown completely apart in a blast so powerful that windows in nearby buildings were also shattered, according to video footage from a South Korean surveillance camera on the border.
The South’s Unification Ministry confirmed that North Korea had demolished the four-story glass-and-steel building that housed what had been known as the joint liaison office. Hours later, the North’s official news agency said “the liaison office was tragically ruined with a terrific explosion,” adding that the action reflected “the mind-set of the enraged people” of North Korea.
No South Koreans had worked at the office since January, when it was closed because of the coronavirus pandemic.
The office, staffed by personnel from both sides, was opened in 2018, at a time when the North Korean leader, Kim Jong-un, and President Moon Jae-in of South Korea had held optimistic meetings and were discussing the possibility of broad economic cooperation.
It was the first channel for full-time, person-to-person contact between the Koreas, which have technically been at war for decades because an armistice, not a peace treaty, halted the Korean War in 1953. South Korea had considered the office an important step toward ending decades of enmity, hoping it would eventually lead to the establishment of diplomatic missions in each other’s capitals.
But relations between the Koreas have soured since then, and this month, North Korea began making the liaison office a rhetorical target. On June 5, it threatened to close it down. Four days later, it cut off all communication lines with the South, including one that went through the liaison office. The North said it was determined to “completely shut down all contact means with South Korea and get rid of unnecessary things.”
Three days before the demolition, Kim Yo-jong, a sister and prominent adviser to Mr. Kim, had warned that “before long, a tragic scene of the useless North-South joint liaison office completely collapsed would be seen.”
For weeks, the North has been threatening to walk away from the more cordial relationship it established with the South in 2018. It reacted with anger this month to propaganda campaigns carried out by activists in South Korea, who have used balloons to send leaflets over the border denouncing Mr. Kim and his repressive government. South Korea, hoping to keep the peace, has vowed to stop the balloon launches and is planning legislation that would outlaw them.
Last week, the North referred to the South as an “enemy.” And on Tuesday, hours before the demolition in Kaesong, the North’s military had threatened to send back troops that it had previously withdrawn from areas near the South Korean border.
The North Korean People’s Army said it had been asked to develop “an action plan” to “turn the front line into a fortress and further heighten the military vigilance against the South,” according to a statement published by the state media. It said the plan would involve returning soldiers to areas that had been demilitarized under past agreements with the South.
Sending more troops to the border — already the most heavily fortified in the world — would further raise tensions with the South. But in saying that the move was in the planning stages, the North seemed to be leaving room for compromise.
The South Korean military warned on Tuesday that it would “respond strongly” to any provocative action by the North along the border.
The threatened North Korean troop deployment would involve areas near the border that have been demilitarized since 2000, when the two Koreas’ leaders met for the first time. Monday was the 20th anniversary of that summit meeting.
Under those agreements, the North withdrew some of its frontier military units to make way for roads linking South Korea to Diamond Mountain — a resort destination in the North, which became the site of an experiment in inter-Korean tourism — and to Kaesong, where the two Koreas jointly operated an industrial park years before the liaison office opened.
Both projects were part of the South’s “Sunshine Policy” of improving ties through economic cooperation, which led to the 2000 meeting between Mr. Kim’s father, Kim Jong-il, and then-President Kim Dae-jung of the South. But that good will soured over the years as North Korea continued to develop a nuclear arsenal, and the two projects were eventually shut down.
The Koreas’ relations warmed again in 2018. Kim Jong-un and Mr. Moon agreed to stop cross-border propaganda, and they set a goal of resuming the Kaesong industrial park and the Diamond Mountain project. They also removed more troops from the border area, shutting down some of the guard posts that both Koreas maintain within the so-called Demilitarized Zone that separates them.
But acrimony has returned in recent months. Mr. Kim’s diplomacy with President Trump collapsed last year, frustrating his hopes of winning relief from tough international sanctions imposed on the North over its nuclear weapons. He has since stepped up pressure on the South to move ahead with the Kaesong and Diamond Mountain ventures, both of which had brought the North much-needed hard currency.
Under the 2018 agreements, however, those joint projects were to be resumed only as part of a broader deal to denuclearize the North. The South’s refusal to proceed with them regardless has led to increasingly harsh rhetoric from the North, whose economy, already suffering under the sanctions, has been damaged further by the coronavirus pandemic.
On Saturday, Ms. Kim, Mr. Kim’s sister, said the North should no longer “trust the trite language” coming from Mr. Moon’s government.
“I feel it is high time to surely break with the South Korean authorities,” she said, adding that the “next step” would be taken by the North Korean military.