Seoul - North Korean leader Kim Jong-un has ordered his frontline troops onto a war-footing against a backdrop of rising military tensions with South Korea.
The announcement follows a rare exchange of artillery shells across the two countries' heavily fortified border.
The mine-strewn Demilitarised Zone (DMZ) is a legacy of the 1950-53 Korean War, which ended with an armistice, not a peace treaty, leaving the Korean Peninsula still technically in a state of war.
The North's official KCNA news agency said the move came during an emergency meeting late on Thursday of the powerful Central Military Commission of which Kim is the chairperson.
During the meeting, Kim ordered frontline, combined units of the Korean People's Army (KPA) to "enter a wartime state" from Friday, 17:00.
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 CMC meeting came hours after the two Koreas traded artillery fire on Thursday, leaving no apparent casualties but pushing already elevated cross-border tensions to dangerously high levels.
The KPA followed up with an ultimatum sent via military hotline that gave the South 48 hours to dismantle loudspeakers blasting propaganda messages across the border or face further military action.
The ultimatum expires on Saturday at 17:00.
The South's defence ministry dismissed the threat and said the broadcasts would continue.
The CMC backed the army's ultimatum and also ratified plans for "a retaliatory strike and counterattack on the whole length of the front", KCNA said.
There was no immediate response from South Korea, but the unification ministry announced it was restricting access to the North-South's joint industrial zone at Kaesong.
Only South Koreans with direct business interests in Kaesong - which lies 10km over the border inside North Korea - would be allowed to travel there, a ministry spokesperson said.
The Kaesong industrial estate hosts about 120 South Korean firms employing up to 53 000 North Korean workers and is a vital source of hard currency for the North.
Restricting access will probably be seen as a thinly veiled threat by South Korea to shut the complex down completely if the situation at the border escalates further.
Thursday's artillery exchange in a western quarter of the border came amid heightened tensions following mine blasts that maimed two members of a South Korean border patrol earlier this month and the launch this week of a major South Korea-US military exercise that angered North Korea.
South Korea said the mines were placed by North Korea and responded by resuming propaganda broadcasts across the border, using loudspeakers that had remained silent for more than a decade.
The South Korean military said the North side fired first on Thursday and that it retaliated with dozens of 155mm howitzer rounds.
South Korean troops were placed on maximum alert, while President Park Geun-hye chaired an emergency meeting of her National Security Council and ordered a "stern response" to any further provocations.
The CMC meeting in Pyongyang insisted that the situation would only de-escalate if South Korea turned off the propaganda loudspeakers.
According to the KCNA report, military commanders were despatched to the frontline to prepare "to destroy the means for psychological warfare... and put down possible counter-actions".
The US and UN both said they were following the situation on the Korean peninsula with deep concern.
The US State Department urged North Korea to avoid provoking any further escalation and said it remained "steadfast" in its commitment to defending ally South Korea.