The two Koreas traded fire on Thursday after the North launched artillery shells apparently targeting propaganda loudspeakers installed across the border, threatening additional military action against the broadcasts and further heightening tension on the peninsula.

The North fired a 14.5-milimeter anti-aircraft gun once at around 3:53 p.m. at a town in Yeoncheon County, Gyeonggi Province, and then a 76.2-mm direct fire weapon “several times” at 4:15 p.m. within the Demilitarized Zone, military officials here said. Shortly after detection, the Army discharged a 155-milimeter self-propelled gun at 5:04 p.m.

The Army reported no damage. Casualties in the North were not immediately known.

Coincidentally with the exchange of fire, Pyongyang sent two separate letters to the South, calling for a withdrawal of the loudspeakers and threatening military action.

In the letter sent from the General Staff of the North Korean People’s Army to South Korea’s Defense Ministry through a border telephone channel at around 4:50 p.m., Pyongyang warned that it would initiate “military action” unless Seoul stops the propaganda broadcasts within 48 hours from 5 p.m.

In a separate letter by Kim Yang-gon, director of the United Front Department in charge of cross-border affairs, to National Security Office chief Kim Kwan-jin, Pyongyang said the broadcasts constitute a “declaration of war” but it is willing to resolve the current situation and “open a way out for the improvement of the relationship.”

Seoul said it has no plan to dismantle the speakers at this point.

“The letter, which coincided with the provocation against us, was most likely to be intended to cover up the fact that the aggravated situation resulted from North Korea’s land mine provocation,” the Unification Ministry said in a statement.

“We will solidly take measures under the principle that the government sternly responds to North Korea’s wrong behavior.”

President Park Geun-hye convened an emergency meeting of the presidential National Security Council to examine the incident and formulate follow-up steps.

The government also released an evacuation order for some residents of Yeoncheon, Paju, Ganghwado Island and other border areas.

The North’s anti-aircraft gun ammunition appeared to have hit an uninhabited hill located several kilometers away from a loudspeaker set, an official at Seoul’s Defense Ministry said. Yet the North did not appear to have aimed at the equipment, he noted, without elaborating.

“We detected signs that the North Korean military staged two rounds of firearm provocation in the southern part of the Military Demarcation Line, and fired dozens of rounds of a 155-millimeter self-propelled gun as warning shots,” Joint Chief of Staff spokesman Col. Jeon Ha-kyu said at a news briefing.

“Our military immediately raised our security posture to the top-notch, while closely monitoring the North Korean military’s movement and maintaining full readiness to respond robustly and firmly to any further provocation.”

Last week, the South Korean military restarted the broadcasts in the Demilitarized Zone, closing an 11-year hiatus, after the JCS found that the North was behind the Aug. 4 explosion of land mines that inflicted serious wounds on two soldiers.

Flatly denying its responsibility, the North Korean military urged to remove the loudspeakers in its “public warning” last Saturday, threatening “indiscriminate strikes” if its demand was not met.

The North, for its part, has resumed its own propaganda program in the area and tightened vigilance against the South.

The Rodong Sinmun, the North’s ruling Workers’ Party’s mouthpiece, called the border broadcasts and anti-Pyongyang leaflet sprayings a “prelude to war” and “most explicit plot for psychological warfare.”

Pyongyang has also been churning out military threats against the ongoing Ulchi Freedom Guardian, a major annual drill by South Korea and the U.S.

“This year’s UFG exercise implies a stringency that cannot be overlooked in light of various unusual signs that have recently emerged around the Military Demarcation Line,” the paper said Tuesday.

Yet tension is set to flare up across the border as Defense Minister Han Min-koo vowed to expand the broadcasts “to full-scale.” During a parliamentary session last week, he said that the program was airing at four spots, and anti-Pyongyang leaflets and propaganda electronic boards are other options being considered as next retaliatory steps.

The minister has also raised the possibility of another major provocation timed for the anniversary of the founding of the Workers’ Party in October.

The loudspeakers were taken down in 2004 as a result of inter-Korean military dialogue. They were reinstalled at 11 places along the MDL following the North’s 2010 fatal attack on the corvette Cheonan, which led Pyongyang to threaten to shoot them down, but had since been idle.

With the absolute majority of North Koreans remaining isolated from the outside world, the broadcasts have proved to be a rare source for news of the execution and purge of elite military executives and other power struggles within the top echelons, information about free democracy and other regional and global issues, along with weather forecasts and music. Many defectors resettled in South Korea have testified that they had come to hear first about the South through the broadcasts, which can travel as far as 10 kilometers.

By Shin Hyon-hee (heeshin@heraldcorp.com)