First there was a rare exchange of artillery shells across North and South Korea's heavily-fortified border and now North Korea's leader Kim Jong-un has ordered his frontline troops onto a war-footing.

North Korea's official KCNA news agency said the decision was made at an emergency meeting of the Central Military Commission held late Thursday (20 August). Kim chairs the commission, AFP reports.

It said at the meeting, Kim ordered the frontline to "enter a wartime state" from 21 August 5.00 pm local time. KCNA quoted him as saying that the troops should be "fully battle ready to launch surprise operations" while the entire frontline should be placed in a "semi-war state."

North Korea had earlier issued an ultimatum, sent via military hotline, that gave South Korea 48 hours to dismantle loudspeakers blasting propaganda messages across the border or face further military action.

South Korea's defence ministry dismissed the threat and said the broadcasts would continue. It has raised its alert status to the highest level. In a televised broadcast, its defence spokesman said: "We're also maintaining our military readiness to strongly respond if there is any further provocation."

South Korean President Park Geun-hye has told defence officials to "react firmly" to North Korean provocation. "Our military has stepped up monitoring and is closely watching North Korean military movements," Seoul's Defence Ministry said.

South Korea has also set up a barricade on the road leading to North Korea's Kaesong joint industrial complex at a military checkpoint in the border city of Paju. The South Korean Unification Ministry announced it was restricting access to the North-South's joint industrial zone at Kaesong.

The zone houses an industrial estate with 120 companies employing 53,000 North Koreans, a vital source of hard currency for North Korea, The move is seen as a thinly veiled threat to shut the complex down completely if the crisis escalates further, AFP reports.

Seoul said North Korean initially fired a single artillery round over the border shortly before 4.00pm local time and minutes later, it fired several more in the direction of one of the loudspeaker units.

The shells however fell short of dimilitarised zone, a 2.5 mile-wide buffer area between both countries.

In a separate letter, North Korea told Seoul that it was willing to end the ongoing conflict over the loudspeaker broadcasts.

The letter sent by North Korea's Kim Yang Gon, secretary of the Central Committee of the Workers' Party of Korea said the broadcasts were a declaration for war but that North Korea "was willing to offer an exit to settle the current situation and improve the relations."

South Korea has evacuated residents in the Yeoncheon county, around 40 miles north of Seoul as a preventive measure, The Telegraph reported.

US expresses concern

The US, which has about 28,500 military personnel stationed in South Korea said it was concerned and was closely monitoring hte situation.

"Such provocative actions heighten tensions, and we call on Pyongyang to refrain from actions and rhetoric that threaten regional peace and security," US State Department spokesperson Katina Adams said.

The US will take "prudent measures" to ensure the well-being of US personnel but it did not elaborate, Reuters reported.

Tensions this time around escalated following landmine explosions at the demilitarised zone wounded two South Korean soldiers. Seoul has accused North Korea of laying the mines which has been denied by Pyongyang.

South Korea then started propaganda broadcasts, prompting North Korea to do the same. An exchange of fire took place on Thursday during the annual joint US and South Korean military exercises.

Both North and South Korea technically remain in a state of war since the 1950-1953 Korean War ended in a truce, not a peace treaty.