(UPDATE) SEOUL — North Korea blew up sections of the deeply symbolic roads connecting it to South Korea on Tuesday, Seoul's military said, adding that it had conducted a "counterfire" operation in response.
Pyongyang's armed forces last week vowed to permanently seal its southern border after spending months laying mines and building anti-tank barriers in the wake of leader Kim Jong Un declaring South Korea his country's "principal enemy."
The North also accused Seoul of using drones to drop anti-regime propaganda leaflets on the capital Pyongyang, with Kim convening a security meeting to direct a plan of "immediate military action" in response, state media reported on Tuesday.
"North Korea has detonated parts of the Gyeongui and Donghae roads north of the Military Demarcation Line (MDL)," the South's Joint Chiefs of Staff said on Tuesday, referring to inter-Korean infrastructure that once connected the two countries.
"There has been no damage to our military, and our forces conducted counterfire in areas south of the MDL," it added.
The roads have long been shuttered, but destroying them sends a clear message that Kim is not prepared to negotiate with the South, experts said.
"This is a practical military measure related to the hostile dual-state system that North Korea has frequently mentioned," Yang Moo-jin, president of the University of North Korean Studies in the capital Seoul, told Agence France-Presse (AFP).
The North may also be looking to erect more physical barriers along the border, Yang said, adding that the road detonations could be "preparatory work for its construction of those walls."
South Korea's military released video footage showing North Korean soldiers in military uniforms setting up what appeared to be cameras on tripods ahead of a huge explosion, which blew up sections of the Gyeongui road.
The explosion emitted thick, billowing smoke and took place amid lush forests. More footage, apparently from after the blasts, showed excavators digging, while North Koreans in military uniforms worked as large red trucks arrived.
Seoul's army also released separate footage showing North Korea blowing up a section of the Donghae road, on the east coast.
South Korea's Unification Ministry slammed the North for what it called an "extremely abnormal" provocation, adding that Seoul had shouldered millions in costs for building the infrastructure.
"North Korea still has repayment obligations regarding this funding," it added.
Seoul's military initially denied sending drones north but has subsequently declined to comment, even as Pyongyang has blamed them directly, warning it would consider it "a declaration of war" if another drone was detected.
Later on Tuesday, Kim's powerful sister Kim Yo Jong released a statement saying Pyongyang has secured "clear evidence" that the South Korean military was behind the drones that reached Pyongyang.
"The provocateurs will have to pay a dear price," she said.
Activist groups in the South have long sent propaganda northward, typically carried by balloons, but enthusiasts are also known to have flown small, hard-to-detect drones into the North.
Unlike conventional drones made of metal, the devices they used were constructed from expanded polypropylene, similar to styrofoam, allowing them to go undetected by both South and North Korean authorities, said enthusiasts who spoke to local media.