The North's KCNA news agency reported that the military had blasted sections of road and rail links with South Korea on Tuesday as legitimate action taken against a hostile state as defined by its constitution.
Sixty-metre sections of the road and railway on its side of the border that had been laid as crossings were now completely blocked as part of a "phased complete separation of its territory" from the South, it said.
"This is an inevitable and legitimate measure taken in keeping with the requirement of the DPRK Constitution which clearly defines the ROK as a hostile state," KCNA said, using the North's official name, the Democratic People's Republic of Korea, and South Korea's, the Republic of Korea.
KCNA cited a defence ministry spokesperson as saying the country would take further steps to "permanently fortify the closed southern border" but did not mention any other changes to the constitution that leader Kim Jong-un had ordered.
In January, Kim called for a constitutional amendment to erase unification as a goal in its ties with the South, accusing Seoul of colluding with the United States to seek the collapse of his communist regime and a clear defining of its territory.
North Korea's Supreme People's Assembly met over two days last week where it had been expected to amend the constitution to officially designate South Korea as a separate country and a main enemy.
State media had not reported on such a move, drawing speculation whether the change to the constitution had been postponed.
South Korea has said its policy was to continue to pursue national unification but respond with force if North Korea mounts any aggression.
Tensions between the rivals have been escalating since last year with both sides declaring an agreement signed in 2018 to ease military tension was no longer valid.
North Korea sharply intensified its hostile rhetoric in recent days, accusing the South of intruding on its airspace by flying drones and vowing retaliation.
South Korea's government has declined to say whether its military or civilians flew the alleged drones.
The South's military fired warning shots south of the border on Tuesday in response to the North's detonations on roads and railways.
Pyongyang said 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.
