Tensions have been rising on the Korean peninsula, with Pyongyang saying last week it would cut off the inter-Korean roads and railways entirely and further fortify the areas on its side of the border as part of its push for a "two-state" system scrapping its longstanding goal of unification.
At around midday on Tuesday, some northern parts of road and rail lines connected to the South were destroyed, the South's Joint Chiefs of Staff (JCS) said.
Seoul's unification ministry, which handles cross-border affairs, condemned the incident as a clear violation of past inter-Korean agreements, calling it "highly abnormal."
"It is deplorable that North Korea is repeatedly conducting such regressive behaviour," ministry spokesman Koo Byoung-sam told a briefing.
In response to the blasts, the South's military fired warning shots south of the military demarcation line, although there was no damage on Seoul's side of the border, it said.
A video released by the South's military showed an explosion and a plume of smoke rising above an area of road where the North had put up a black barrier. On the South Korean side, a road sign read "Goodbye" and specified that the North Korean city of Kaesong was 10 metres ahead.
It also showed several dump trucks and earth-movers approaching with a group of North Korean military officials observing and guiding the vehicles.
The JCS said the North had already been installing landmines and barriers along the border, and warned on Monday that it was getting ready for a detonation.
The South had increased surveillance and readiness after the incident, it said.
OLD SIGNS OF RECONCILIATION
Pyongyang has been taking steps to sever inter-Korean ties, redefining the South as a separate, hostile enemy state, since leader Kim Jong Un declared it a "primary foe" early this year and said unification was no longer possible.
The two Koreas are still technically at war after their 1950-53 war ended in an armistice, not a peace treaty.
The cross-border roads and railways are remnants of rapprochement that included a 2018 summit. Seoul has funnelled more than $132 million for rebuilding the links in the form of cheap loans to Pyongyang, unification ministry data showed.
"It was a major inter-Korean cooperation project that was carried out upon the request from the North," said ministry spokesman Koo, adding Pyongyang was still obliged to repay the loans.
In 2020, the North blew up a joint liaison office set up in a border town after nuclear talks with the United States collapsed.
South Korea filed a lawsuit in 2023 against the North, seeking damages of around 45 billion won ($33 million) from the joint office's demolition.
Tensions mounted after the North last week accused Seoul of sending drones over Pyongyang. North Korea said the drones scattered a "huge number" of anti-North leaflets, and Kim Yo Jong, Kim's powerful sister, warned Seoul of a "horrible disaster."
A South Korean JCS spokesman declined on Monday to answer questions over whether the South Korean military or civilians had flown the alleged drones.
Earlier on Monday, state media KCNA reported that Kim had convened a meeting on Sunday with defence and security officials to discuss responses to the drone intrusion.
