North Korea has reportedly completed preparation for an attack against South Korea, after giving the South a deadline of 48 hours to stop broadcasting anti-Pyongyang propaganda broadcasts.

A report by North Korean state media KCNA said the nation's leader Kim Jong-un had declared a "semi-state of war" at an emergency meeting late on Thursday evening.

The command Kim issued to his sizeable military was to be "fully ready for any military operations at any time," beginning from 5pm local time (4am ET) on Friday, according to KCNA.

This is not the first time this sort of strong rhetoric has been used by the leadership north of the most militarized border in the world. In the 60 years since the Korean War ended, the two countries have never officially been at peace, as an armistice rather than a peace treaty was signed.

Tensions between the two states escalated on Thursday, after an exchange of rocket and artillery fire across the border between them.

North Korea appears to have launched the first attack by firing a shell across the border. South Korean media reported it was likely that the North was aiming at a propaganda-blaring loudspeaker situated in a border town about 35 miles north of Seoul in the western part of the border area.

The South then reportedly retaliated with tens of 155mm artillery rounds targeted towards the location where the shell originated, according to their defense ministry.

"Our military has stepped up monitoring and is closely watching North Korean military movements," the ministry said in a statement.

The South's Defense Ministry also said that the North had threatened "new military action" if the loudspeaker broadcasts were not stopped within 48 hours.

'South Korean gangsters, insane to find provocation against us, again fabricated the incident of North Korea firing towards the South on the afternoon of August 20'

South Korean news agency Yonhap reported that the country ordered civilians to evacuate from the western border, though the country's defense ministry told AFP that they could not immediately confirm the report.

Meanwhile, a North Korean state TV presenter claimed that the North had fired nothing. "South Korean gangsters, insane to find provocation against us, again fabricated the incident of North Korea firing towards the South on the afternoon of August 20," she said. "Using the incident as an excuse, they committed a reckless military act in firing several rounds towards our territory."

The exchange of fire came as US and South Korea were conducting their annual joint military exercises.

Beginning on Monday and titled the "Ulchi Freedom Guardian," the war games simulate the invasion of the South by the North, and involve around 80,000 South Korean and US troops, as well as countries including the UK, France, Australia, Canada, Colombia, Denmark, and New Zealand.

"Such large-scale joint military exercises... are little short of a declaration of a war," North Korea's Committee for the Peaceful Reunification of Korea, which oversees cross-border issues, said in response to the exercises last week, adding that they could even lead to "all out conflict."

Earlier this month South Korea promised "searing" consequences for the maiming of two of their soldiers, who officials said were victims of land mines planted on the border. This included the resumption of propaganda broadcasts across the border towards rival North Korea for the first time since 2004.

The North's National Defense Commission (NDC) denied the accusations that they had planted land mines, calling them "absurd."

"If our army really needed to achieve a military purpose, we would have used strong firearms, not three mines," the NDC said in a statement carried by the KCNA news agency.