The South Korean Unification Ministry—which focuses on promoting reunification with the North—confirmed that the liaison office in the North Korean border city of Kaesong was destroyed on Tuesday, Reuters reported.

Photos taken from the South Korean border city of Paju showed a plume of smoke rising above the Kaesong Industrial Complex, where the inter-Korean liaison office was located.

China’s state-controlled CCTV channel said that Chinese Foreign Ministry spokesperson Zhao Lijian urged calm at his daily press briefing in Beijing.

China is North Korea’s main ally and Pyongyang has historically been heavily dependent on Beijing for economic and diplomatic support. However, the Chinese government has condemned more aggressive action by North Korea, for example nuclear weapon tests and the sinking of a South Korean warship in 2010.

Zhao told reporters that North and South Korea are part of the same nation, and that as a close neighbor China has always wished for peace and stability on the peninsula, CCTV reported.

Reuters cited an anonymous South Korean military source who said there were indications that the North was planning to destroy the liaison office, and that South Korean military officials watched live surveillance of the building as it was blown up.

North-South relations have deteriorated in recent weeks over anti-Pyongyang leaflets floated into North Korea from the South by defector groups seeking to undermine Kim Jong Un’s regime. Leaflets typically detail the human rights abuses committed by the regime, as well as money, food, USB sticks carrying South Korean entertainment, and other goods.

Earlier this month, the South Korean government said it would ban the balloons as an effort to reduce tensions. The decision was made soon after Kim’s sister Kim Yo Jong had branded balloon senders “human scum.”

North Korean media reported that weekend that Kim Yo Jong—a senior official within the ruling Workers’ Party and considered a key advisor to her older brother—had ordered the North’s inter-Korean affairs department to “decisively carry out the next action.”

She also threatened that “before long, a tragic scene of the useless north-south joint liaison office completely collapsed would be seen.”

The office was set up in 2018 to encourage North-South cooperation after a period of high tensions when both Pyongyang and Washington, D.C. were mooting military action. The office has been closed since January due to concerns over the coronavirus pandemic.

Also on Tuesday, North Korean state media said the military had been considering an “action plan” to re-occupy areas demilitarized through a 2018 inter-Korean agreement. State media said its forces would “turn the front line into a fortress.”

South Korea’s defense ministry called on the North to stick to the 2018 agreement, which prompted the dismantling of fortifications in the border Demilitarized Zone and a commitment to end “all hostile acts.”

Defense ministry spokesperson Choi Hyun-soo said Seoul was “taking the situation seriously” and that the “military is maintaining readiness posture to be able to respond to any situation,” Reuters reported.