North Korean leader Kim Jong-Un ordered his frontline troops onto a war footing from Friday, as military tensions with South Korea soared.

North Korean leader Kim Jong-Un ordered his frontline troops onto a war footing from Friday, as military tensions with South Korea soared following a rare exchange of artillery shells across their heavily fortified border.

South Korean forces are already on maximum alert after the North's military issued an ultimatum on Thursday for Seoul to switch off loudspeakers blasting propaganda over the border within 48 hours or face concerted military action.

An emergency meeting late Thursday of the North's powerful Central Military Commission (CMC) -- chaired by Kim Jong-Un -- endorsed the ultimatum and ratified plans for "a retaliatory strike and counterattack on the whole length of the front".

According to the official KCNA news agency, Kim ordered frontline, combined units of the Korean People's Army (KPA) to "enter a wartime state" from Friday 5:00 pm (0800 GMT).

The troops should be "fully battle ready to launch surprise operations" while the entire frontline should be placed in a "semi-war state", KCNA quoted him as saying.

The South Korean Joint Chiefs of Staff responded with a direct message to the KPA, urging it to refrain from any "reckless acts" and warning that it would react strongly to any further provocation.

North Korea has made similar announcements at times of high tension in the past, most recently in 2013 when Kim declared "a state of war" with the South.

Technically, the two Koreas have been at war for the past 65 years, as the 1950-53 Korean War ended with a ceasefire that was never ratified by a formal peace treaty.