North Korea said it will permanently block its border with South Korea and boost its front-line defence posture to cope with "confrontational hysteria" by South Korean and US forces.

While the move was 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 will "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 (KCNA).

The North's military called its steps a "self-defensive measure for inhibiting war and defending the security" of North Korea.

It said that "the hostile forces are getting ever more reckless in their confrontational hysteria." 

It 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 misjudgement and potential accidental clashes.

South Korea's Defense Ministry said it confirmed the North Korean message with the American-led UN Command. It said South Korea is closely communicating and coordinating with the UN Command but didn't 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 front-line security posture and prevent its soldiers and citizens from defecting to South Korea.

In a report to parliament on Tuesday, South Korea's Unification Ministry said that North Korea has been removing ties on the northern side of cross-border railways and nearby lamps and planting mines along the border.

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 didn't 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.

At the centre of outside attention was whether North Korea makes new legal claims on the waters currently controlled by South Korea off their west coast.

The poorly marked western sea boundary is where three bloody naval skirmishes and two deadly attacks blamed on North Korea happened in the past 25 years.

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.

Mr 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 Mr Kim likely aims to diminish South Korea's voice in the regional nuclear stand-off and seek direct dealings with the US. They say Mr Kim also likely hopes to diminish South Korean cultural influence and bolster his rule at home.

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.

AP