Pyongyang said it would "cut off roads and railways" that would once facilitate travel between the two Koreas.
While North Korea described the move as a "major military step," an analyst suggested it was likely a continuation of a process that began long ago.
Inter-Korean relations are at one of their lowest points in years, with Pyongyang shutting down agencies dedicated to unification and declaring South Korea its "main enemy."
The nuclear-armed country was expected to abandon a historic inter-Korean agreement signed in 1991 at a key parliamentary session that ended on October 8, part of leader Kim Jong Un's drive to formally designate the South as an enemy state.
But in a report today revealing the appointment of a new defense chief, official state media made no mention of the pact's termination.
Hours later, however, the army said it was planning a "significant military step" that would "completely cut off roads and railways connected to the ROK (South Korea) and fortify the relevant areas on our side with strong defense structures."
It added that it had sent a telephone message to US forces to "prevent any misjudgment and accidental conflict in relation to the reinforcement project to be launched in the acute zone of the southern border".
Although the border is one of the most heavily militarized in the world, in August it failed to prevent a North Korean soldier from crossing into the South.
In July, Seoul said Pyongyang had spent months planting landmines and erecting barriers while turning the area into a wasteland along the heavily fortified border.
In June, the South Korean military reported that North Korean soldiers tasked with fortifying the border had suffered "numerous casualties" as a result of mine explosion incidents.
In the same month, Seoul's spy agency said it had detected signs that North Korea was destroying sections of a railway linking the two Koreas.
"North Korea is already destroying parts of the Donghae Line railway, apparently with the intention of completely severing its link with the South," said Yang Mu-jin, president of Seoul's North Korea Studies University, calling the latest statement "official confirmation."
The North said its decision was a "self-defensive measure" in response to South Korea's "military exercises" and visits by US strategic nuclear assets.
Although no constitutional changes involving Seoul were reported at this week's meeting, Hong Min, a senior analyst at the Korea Institute for National Unification, said North Korea may wait for the results of next month's U.S. election before taking any step.
Pyongyang also appointed Noh Kwang Chol as the new defence minister to replace Kang Sun Nam.
The announcement of Noh's appointment comes a day after Seoul's defence chief reported that North Korean soldiers were likely fighting in Ukraine alongside Russian troops, with some believed to have already been killed. | BGNES
