SEOUL (Reuters) – North Korea on Tuesday blew up a joint liaison office set up in a border town in 2018 to foster better ties with South Korea after threatening action if defectors continued with a campaign sending propaganda leaflets into the North.

The liaison office in Kaesong – a gleaming blue-glass four-storey structure in an otherwise drab industrial city – was “ruined with a terrific explosion,” North Korea’s state news agency KCNA said.

The destruction of the building, which had been closed since January due to the coronavirus fears, was a major setback to efforts by South Korean President Moon Jae-in to coax North Korea into cooperation.

It also appeared to be a further blow to U.S. President Donald Trump’s hopes of persuading North Korea to give up its nuclear weapons and open up to the outside world.

A spokesman for the U.S. State Department said the United States fully supports Seoul’s efforts on inter-Korean relations and urged Pyongyang to “refrain from further counterproductive actions.”

North Korea last week warned the United States to refrain from commenting on inter-Korean affairs if it wanted its November presidential election to go smoothly, raising concerns it could be contemplating a return to nuclear and long range missile testing.

Trump has hailed Pyongyang’s freeze in such testing as a win from his unprecedented but otherwise fruitless series of meetings with North Korean leader Kim Jong Un in 2018 and 2019.

Earlier, the State Department said Deputy Secretary of State Stephen Biegun, the top U.S. official dealing with North Korea, would travel with Secretary of State Mike Pompeo to Hawaii on Tuesday.

Sources said Pompeo will hold talks with China’s top diplomat Yang Jiechi in Hawaii on Wednesday on issues including North Korea.

China is North Korea’s main ally and neighbour and shares U.S. concerns about Pyongyang’s weapons programs. U.S. officials have stressed the need for Beijing to strictly enforce international sanctions on North Korea.

Russia said it was concerned about the situation on the Korean peninsula and called for restraint from all sides.

Surveillance video from South Korea’s defence ministry showed a large explosion that appeared to bring down the office building. It also appeared to cause a partial collapse of a neighbouring 15-storey building that had housed South Korean officials who had staffed the liaison office.

South Korea’s national security council said Seoul would respond sternly if North Korea continued to raise tensions.

The destruction of the building “broke the expectations of all people who hope for the development of inter-Korean relations and lasting peace on the peninsula”, deputy national security advisor Kim You-geun told a briefing.

The first diplomatic mission of its kind, the liaison office was established as part of a series of projects aimed at reducing inter-Korean tensions.

South Korean vice unification minister Suh Ho, who co-headed the liaison office, called the demolition “unprecedented in inter-Korean relations” and a “nonsensical act”.

Reclusive North Korea and the democratic South remain technically at war because their 1950-53 conflict ended in a truce, not a treaty.

Tensions have been rising over recent days with North Korea threatening to cut ties and retaliate over the propaganda leaflets carrying messages critical of Kim Jong Un.

KCNA said the office was blown up to force “human scum and those, who have sheltered the scum, to pay dearly for their crimes”.

Several defector-led groups, which North Korea refers to as “human scum”, have regularly sent flyers over the border, together with food, $1 bills, mini radios and USB sticks containing South Korean dramas and news, usually by balloon or in bottles by river.

‘TRAGIC SCENE’

The Kaesong building had originally been used as offices in the Kaesong Industrial Complex, a joint venture between the two Koreas suspended in 2016 amid disagreement over North Korea’s nuclear and missile programmes.

South Korea spent at least 9.78 billion won ($8.6 million) to renovate it in 2018. South Koreans worked on the second floor and North Koreans on the fourth floor. The third floor housed conference rooms for meetings between the two sides.

North Korean state media reported on Saturday that Kim Jong Un’s sister Kim Yo Jong had ordered the department in charge of inter-Korean affairs to “decisively carry out the next action”.

“Before long, a tragic scene of the useless north-south joint liaison office completely collapsed would be seen,” she was reported as saying.

North Korean media also quoted the military as saying it had been studying an “action plan” to re-enter zones that had been demilitarized under the 2018 pact and “turn the front line into a fortress”.

South Korea’s defence ministry urged North Korea to abide by the agreement, under which both sides vowed to cease “all hostile acts” and dismantled a number of structures along the heavily fortified Demilitarized Zone between the countries.