SEOUL — North Korea on Tuesday blew up a liaison office it operated with South Korea and threatened to move troops into the demilitarized zone, dramatically ramping up military tensions on the peninsula as the nuclear-armed regime seeks concessions from Seoul and Washington.

The joint facility in the North Korean border city of Kaesong — which the two sides opened in 2018 as a de facto embassy in the absence of formal diplomatic ties — was demolished just before 3 p.m. local time, the South's Unification Ministry said.

Shortly afterward, as smoke billowed near the heavily defended frontier, North Korea's state media said the liaison office was "tragically ruined with a terrific explosion."

North Korean state TV aired video of the destruction of the inter-Korean liaison office in Kaesong, North Korea on June 16. (Reuters)

The destruction of a rare symbol of cross-border cooperation marked a sharp escalation from Pyongyang, which has sounded an increasingly aggressive tone toward South Korea in recent weeks amid a deadlock in diplomacy with the United States. Earlier Tuesday, North Korea's army, one of the world's largest, warned that it was ready to move forces back into border zones the two Koreas had previously agreed to demilitarize.

The regime also has been criticizing plans by defectors in the South to launch pro-democracy leaflets across the border that are intended to promote human rights and undermine the dictatorship of Kim Jong Un.

Demolishing the building was the act of "enraged people" retaliating against "human scum," Pyongyang's official Korean Central News Agency said, an apparent reference to the North Korean defectors.

South Korea expressed "severe regret" at the North's move. Kim You-geun, first deputy chief of Seoul's National Security Council, said it "betrayed the hopes of peace on the Korean Peninsula."

"North Korea will obviously be held accountable for any and all consequences of this incident," Kim said, adding that the U.S. ally would take strong actions if the North carries out further provocations.

Pyongyang's recent outbursts have coincided with the elevation of Kim Yo Jong, the North Korean leader's younger sister, to a more public role in the regime, fueling speculation among political analysts that officials are readying her for higher office.

In a statement Saturday, she labeled the liaison office "useless" and threatened to destroy it "before long," adding that she had asked the military to prepare a "hostile action" to unnerve the South.

"If the South Korean authorities have now [the] capability and courage to carry out at once the thing they have failed to do for the past two years, why are the north-south relations still in stalemate?" she said.

The remarks demonstrated North Korea's frustration over its inability to win relief from international sanctions after two years of detente and nuclear diplomacy with the United States, including three meetings with President Trump.

North Korea blew up an office set up to foster better ties with South Korea June 16 in response to North Korean defectors sending leaflets into the North. (Reuters)

With its trade still severely curtailed, North Korea has been unable to develop its economy, while South Korea's center-left government has been unable to move forward with proposals for joint projects that might boost cooperation with its impoverished neighbor, such as cross-border railways.

Leif-Eric Easley, a professor of international studies at Ewha Womans University in Seoul, said that North Korea is strategically pressuring the South to try to secure concessions on sanctions.

"It's hard to see how such behavior will help the Kim regime get what it wants from the world, but clearly such images will be used for domestic propaganda," Easley said. "So Seoul needs to impose additional costs demonstrating to Pyongyang that its threats are counterproductive."

Earlier this month, North Korea shut down telephone hotlines with South Korea and threatened to scrap a military agreement unless Seoul stopped the anti-Pyongyang activists. South Korea's government said it would ban the activist groups from sending leaflets and asked the police last week to investigate them.

The liaison office, where officials from the two Koreas were able to communicate around-the-clock, had been temporarily closed since January due to concerns over the novel coronavirus outbreak.

But its destruction is a setback for South Korean President Moon Jae-in, who has made efforts to coax Pyongyang to give up nuclear weapons and end deep-rooted hostilities between the rival Koreas the hallmarks of his presidency. The two nations have remained technically at war since their 1953 armistice.

"North Korea should not cut off communications, create tensions and go back to the past era of confrontations," Moon said in a speech Monday.

The uptick in aggressive behavior from Pyongyang is also unfolding at a time of heightened insecurity in Asia, where China has been forcefully asserting its influence amid concern about the Trump administration's commitments to long-standing U.S. alliances.

On Monday, Chinese and Indian forces clashed at their disputed border in the Himalayas in their most serious conflict in decades, leading to fatalities. China has been ramping up threats against Taiwan, the self-governed democracy that Beijing regards as its own territory, tightening its grip over Hong Kong and entrenching control of islands in the disputed South China Sea.

Chinese Foreign Ministry spokesman Zhao Lijian said Tuesday that Beijing hoped for peace and stability on the Korean Peninsula.