North Korea's army said on Wednesday it was moving to "permanently shut off and block the southern border" with the South and had informed the US military to prevent an accidental clash.
Pyongyang said in a statement it would "cut off roads and railways" that might have made travel between the two Koreas possible.
However, it was largely a symbolic gesture because cross-border exchanges and travel between North and South Korea have been halted for years.
Inter-Korean relations are at one of their lowest points in years, with Pyongyang closing agencies dedicated to reunification and declaring South Korea its "principal enemy".
Some analysts thought the announcement could be a potential first step towards more serious action, such as amending the North's constitution to declare a new maritime border south of the current de facto line.
The nuclear-armed North had been expected to scrap a landmark inter-Korean agreement signed in 1991 at a key parliamentary meeting that ended on Tuesday, part of leader Kim Jong Un's drive to officially define the South as an enemy state.
However, state media made no mention of such action in a report on Wednesday announcing a new defense chief.
The army said hours later 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 defense structures".
It said it had sent a telephone message to US forces to "prevent any misjudgement and accidental conflict".
The border between the two Koreas is one of the most heavily militarised in the world, although it failed to prevent a North Korean from crossing to the South in August.
Seoul said in July that Pyongyang had spent months laying landmines and erecting barriers, turning the area into a wasteland.
The South Korean military said a month earlier North Korean soldiers had suffered "multiple casualties" from landmine explosions in the area.
Seoul's spy agency also said in June it had detected signs that North Korea was demolishing sections of a railway line connecting the two Koreas.
That demolition was "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.
Yang described Wednesday's statement from the North as "official confirmation".
- Harsher isolation -
The North Korean military described its decision as a "self-defensive measure" in response to South Korean "war exercises" and visits by US strategic nuclear assets.
Its counterpart in the South slammed the announcement as a "desperate measure stemming from the insecurities of the failed Kim Jong Un regime".
The South Korean military said the North's action would "lead to even harsher isolation" and warned it would "never stand idly by" if Pyongyang sought to change "the status quo".
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 announcing any change to its constitution.
Pyongyang also named No Kwang Chol as its new defense minister on Wednesday, replacing Kang Sun Nam.
That announcement came a day after Seoul's defence chief said North Korean soldiers were likely fighting in Ukraine alongside Russian troops, with some believed to have been killed and more expected to be deployed.