One million people have volunteered to defend North Korea if war breaks out with South Korea.

According to the country’s state media "young people across the Democratic People's Republic of Korea are turning out in the sacred war for defending the country with their faith and will to annihilate the enemies".

North Koreans read public copy of the daily newspaper at the platform of a subway station in Pyongyang (AP)

North Korea has mobilised a large number of submarines and doubled its artillery units along the border. A defense ministry spokesman from the South has said an unprecedented number of submarines, thought to be around 70 percent of the North's total fleet, had left their bases and disappeared from Seoul's military radar.

Inter-Korean relations have been strained since the deadly sinking of a South Korean warship in 2010, which Pyongyang denied responsibility for. Tensions sparked up again this month after two South Korean soldiers were wounded by landmines along the border. The North denied laying the mines but days later Seoul began its propaganda broadcasts in random three-hour bursts from loudspeakers, including news reports and K-pop music.

A South Korean amy soldier walks as his colleague soldiers stand guard on Unification Bridge, which leads to the demilitarized zone (AP)

The North’s leader Kim Jong-Un has declared a "quasi-state of war" in front-line areas and set an ultimatum for Seoul to halt its broadcasts after an exchange of artillery fire on Thursday that claimed no casualties but further escalated tension.