Seoul: North Korea's leader Kim Jong-un put his front-line troops on a war footing, as tensions between his nation and South Korea soar following an exchange of artillery fire.

"Kim Jong-un issued an order of the supreme commander of the Korean People's Army (KPA) that the front-line large combined units of the KPA should enter a wartime state to be fully battle ready to launch surprise operations," the North's official KCNA agency reported on Friday. The website for the outlet went offline later on Friday morning.

The North Korean ruling party's central military commission held an emergency meeting late on Thursday after North and South Korea exchanged fire across the demilitarised zone in one of the worst incidents between the two since 2010.