While the moves were likely a pressure tactic, it's unclear how they will affect ties with South Korea since cross-border travel and exchanges have been halted for years.
North Korea's military said it would "completely cut off roads and railways" linked to South Korea and "fortify the relevant areas of our side with strong defence structures", according to the North's official Korean Central News Agency.
The North's military called its steps a "self-defensive measure for inhibiting war and defending the security" of North Korea.
"The hostile forces are getting ever more reckless in their confrontational hysteria," it said.
The North cited what it called various war exercises in South Korea, the deployment of US strategic assets and its rivals' harsh rhetoric.
South Korea's military said later on Wednesday it would not tolerate any attempt by North Korea to change the status quo.
It said South Korea would "overwhelmingly punish" North Korea if it launched provocations.
A South Korean military statement said North Korea's nuclear and missile programs had threatened peace on the Korean Peninsula.
South Korean officials earlier said North Korea had already been adding anti-tank barriers and reinforcing roads on its side of the border since April in a likely attempt to boost its frontline security posture and prevent its soldiers and citizens defecting to South Korea.
In a report to parliament on Tuesday, South Korea's unification ministry said North Korea had been removing ties on the northern side of cross-border railways and nearby lamps and planting mines along the border.
KCNA said earlier on Wednesday the Supreme People's Assembly met for two days this week to amend the legal ages of North Koreans for working and taking part in elections.
But it did not say whether the meeting dealt with leader Kim Jong-un's order in January to rewrite the constitution to remove the goal of a peaceful Korean unification, formally designate South Korea as the country's "invariable principal enemy" and define the North's sovereign, territorial sphere.
Kim's order stunned many North Korea watchers because it was seen as breaking away with his predecessors' long-cherished dreams of achieving a unified Korea on the North's terms.
Experts say Kim likely aims to diminish South Korea's voice in the regional nuclear stand-off and seek direct dealings with the US.
Tensions on the Korean Peninsula are at their highest point in years, with North Korea continuing a run of provocative weapons tests and South Korea and the US expanding their military drills.
KCNA said North Korea on Tuesday tested a long-range artillery system that observers say pose a direct threat to Seoul, the South Korean capital, which is only an hour's drive from the border.
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