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 on Wednesday.
The military cited what it called various war exercises in South Korea, the deployment of US strategic assets and its rivals' harsh rhetoric.
North Korea said it sent a message to the US military to explain its fortification activity to prevent any misjudgment and potential accidental clashes.
South Korea's defence ministry said it confirmed the North Korean message with the American-led UN Command.
It said South Korea was closely communicating and co-ordinating with the UN Command but did not elaborate.
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 from defecting to South Korea.
KCNA earlier on Wednesday said the Supreme People's Assembly met for two days this week to amend the legal ages of North Koreans for working and participating 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.
Some experts say North Korea might have delayed the constitutional revision but others speculated it amended the constitution without announcing it because of its sensitivity.
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.
Kim also likely hopes to diminish South Korean cultural influence and bolster his rule at home, they say.
