Kim Jong-un has ordered the North Korean frontline troops to be on a war footing after an exchange of fire with South Korea across their heavily fortified border, state media reports.

The North Korean leader declared a “semi-state of war” at an emergency meeting on August 20, the KCN reports.

North Korea threatened action unless Seoul ends its anti-Pyongyang border broadcasts.

The secretive country often uses fierce rhetoric when tensions rise and it has made similar declarations before.

At the emergency meeting of the central military commission, Kim Jong-un had ordered that troops be “fully ready for any military operations at any time” from August 21 at 17:00 local time, the KCNA reports.

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Earlier, North Korea warned that it would take strong military action if South Korea does not end border propaganda broadcasts and dismantles the broadcast facilities “within 48 hours”.

However, in a separate letter Pyongyang said it was willing to resolve the issue even though it considers the broadcasts a declaration of war, South Korea’s unification ministry said, according to Reuters.

The tensions were ratcheted up after North Korea on August 20 shelled across the border reportedly to protest against the propaganda broadcasts which restarted after a hiatus of 11 years.

In 2004, South Korea and North Korea reached an agreement to dismantle their propaganda loudspeakers at the border.

The broadcasts were part of a program of psychological warfare, according to South Korean newspaper Korea Times, to deliver outside news so that North Korean soldiers and border-area residents could hear it.

On August 10, South Korea restarted broadcasting in an apparent reaction to two South Korean soldiers being injured in a landmine explosion in the demilitarized zone that was blamed on North Korea.

Military authorities say days later North Korea also restarted its broadcasting of anti-South propaganda.

However, some reports said that the quality of the North Korean loudspeakers is so bad that it is difficult to understand what they are saying.

South Korea responded with artillery fire. There were no reported casualties.

Meanwhile, South Korea ordered the evacuation of residents from an area of its western border.

South Korea and the US also began annual joint military exercises on August 17 – they describe the drills as defensive, but North Korea calls them a rehearsal for invasion.