North Korea on Tuesday blew up an inter-Korean liaison office in the country's border city of Kaesong in a major escalation of its stance toward its southern neighbor.

North Korean leader Kim Jong Un's sister, Kim Yo Jong, had predicted the destruction of the office in retaliation for anti-Pyongyang leaflets sent by balloon across the border by defector groups and other activists.

South Korea's Unification Ministry said the joint liaison office, a four-story concrete building located in the Kaesong Industrial Zone, was blown up at 2:49 p.m.

Later Tuesday, North Korea's state-run media also reported that the office, one of the symbols of inter-Korean reconciliation, had been "completely ruined."

Photo taken on Sept. 14, 2018, shows the opening ceremony for the inter-Korean liaison office in Kaesong, North Korea. (Kyodo)

Related coverage:

North Korea army put on high alert for return to Demilitarized Zone

Kim Jong Un's sister predicts destruction of joint liaison office

North Korea slams Seoul's call for restart of denuclearization talks

The demolition came in the wake of "corresponding to the mindset of the enraged people to surely force human scum and those, who have sheltered the scum, to pay dearly for their crimes," the official Korean Central News Agency said.

"At 14:50, the liaison office was tragically ruined with a terrific explosion," it added.

Kim You Geun, deputy director of South Korea's National Security Council, said following an NSC meeting that Seoul "feels deeply regretful" over the North's unilateral demolition, calling it "an act of betraying everyone's hope for peace on the Korean Peninsula."

He said North Korea will be held solely responsible for whatever situation follows the demolition and warned that if things continue to worsen, the South will take strong countermeasures.

Suh Ho, South Korea's head of the inter-Korean liaison office, held a separate briefing where he called North Korea's act "irrational" and lodged a strong protest.

"North Korea's recent harsh comments and unilateral shutdown of communication (with the South) have terrified not only our people but also the whole world," Suh said, adding that North Korea should be held responsible its behavior.

South Korea's Defense Ministry also released a statement saying the military will strongly react if North Korea carries out military provocations.

Photos taken in the South Korean border city of Paju that were released by South Korea's Yonhap News Agency showed smoke rising from the area of the liaison office.

The office was set up in September 2018 as part of an agreement between South Korean President Moon Jae In and Kim Jong Un at their summit in April that year.

Office operations were halted in January amid the spread of the new coronavirus.

North Korea recently threatened to abolish the office over the leaflets and stopped responding to daily calls to the office from South Korea.

In a statement released Saturday via KCNA, Kim Yo Jong said, "Before long, a tragic scene of the useless north-south joint liaison office completely collapsed would be seen."

Earlier Tuesday, North Korea warned it had been preparing to send troops to the Demilitarized Zone dividing it from South Korea.

The General Staff of the Korean People's Army said in a statement it would study "an action plan for the army to advance into zones that had been demilitarized under the North-South agreement."