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

A report said Mr Jong-un,declared a “semi-state of war” at an emergency meeting late Thursday.

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

South Korean Vice Defence Minister, Baek Seung-joo, said it was likely the North would fire at some of the 11 sites where the loudspeakers were set up on the South’s side of the Demilitarised Zone (DMZ) separating the countries.

Tension escalated on Thursday when North Korea fired four shells into South Korea, according to Seoul, in apparent protest against the broadcasts. The South fired back 29 artillery shells. Pyongyang accused the South of inventing a pretext to fire into the North.

Both sides said there were no casualties or damage in their territory, an indication that the rounds were just warning shots.

“The fact that both sides’ shells didn’t damage anything means they did not want to spread an armed clash. There is always a chance for war, but that chance is very, very low,” said Yang Moo-jin, professor at the University of North Korean Studies in Seoul.

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

KCNA reported that Mr Jong-un had ordered that troops be “fully ready for any military operations at any time” from 17:00 Friday local time  at the emergency meeting of the central military commission.

South Korea’s Vice Defence Minister said that 11 sites with loudspeakers for the anti-Pyongyang broadcasts are likely to be targets.