North Korea's army said Wednesday it was moving to "permanently shut off and block the southern border" with Seoul and had informed the US military to prevent an accidental clash.
In a statement, Pyongyang said it would "cut off roads and railways" that might have someday facilitated travel between the two Koreas.
While North Korea on Wednesday described the move as a "major military step", an analyst suggested it was likely the continuation of a process long under way.
Inter-Korean relations are at one of their lowest points in years, with Pyongyang shuttering agencies dedicated to reunification and declaring South Korea its "principal enemy".
The nuclear-armed country had been expected to scrap a landmark inter-Korean agreement signed in 1991 at a key parliamentary meeting that ended Tuesday, part of leader Kim Jong Un's drive to officially define the South as an enemy state.
But in a Wednesday report that revealed the naming of a new defence chief, official state media made no mention of the pact being ended.
Hours later, however, the army said it planned "a substantial military step" that would "completely cut off roads and railways connected to the ROK (South Korea) and fortify the relevant areas of our side with strong defence structures."
It added it had sent a telephone message to US forces Wednesday morning to "prevent any misjudgment and accidental conflict over the fortification project to be launched in the acute southern border area".
While the border is one of the most heavily militarised in the world, it failed to prevent a North Korean from crossing it to the South in August.
Seoul said in July that Pyongyang had spent months laying landmines and erecting barriers while turning the area into a wasteland along the heavily fortified border.
In June, the South Korean military said North Korean soldiers tasked with reinforcing the border had suffered "multiple casualties" from landmine explosion incidents.
That same month, Seoul's spy agency said it had detected signs that North Korea was demolishing sections of a railway line connecting the two Koreas.
"North Korea has already been demolishing parts of the Donghae Line railway, seemingly with the intention of completely severing its connection to the South," Yang Moo-jin, president of the University of North Korean Studies in Seoul, told AFP, calling its latest statement "official confirmation".
The North Korean army said Wednesday its decision was a "self-defensive measure" in response to South Korean "war exercises" and visits by US strategic nuclear assets.
While no constitutional revisions involving Seoul were reported at this week's meeting, Hong Min, a senior analyst at the Korea Institute for National Unification, said North Korea could be waiting for the results of next month's US election before making a move.
Pyongyang on Wednesday also named No Kwang Chol as its new defence minister, replacing Kang Sun Nam.
The announcement of No's appointment comes a day after Seoul's defence chief said North Korean soldiers were likely fighting in Ukraine alongside Russian troops, with some believed to have already been killed and more expected to be deployed.
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