South Korea has fired dozens artillery rounds towards North Korea after its neighbour fired several projectiles towards a South Korean loudspeaker that had been blaring anti-Pyongyang broadcasts.

The North was backing up an earlier threat to attack South Korean border loudspeakers that, after a lull of 11 years, have started broadcasting anti-Pyongyang propaganda.

The broadcasts began after South Korea accused the North of planting land mines that maimed two South Korean soldiers earlier this month.

North Korea first fired a single round believed to be from an anti- aircraft gun, which landed at a South Korean border town on Thursday afternoon. About 20 minutes later, several more artillery shells fell on the southern side of the Demilitarized Zone dividing the two Koreas.

South Korea responded with dozens of 155-milimeter artillery rounds, according to South Korean defense officials.

There were no reports of casualties, and North Korea didn’t respond militarily to South Korea’s artillery barrage. But the North’s army later warned in a message that it will take further military action within 48 hours if South Korea doesn’t pull down the loudspeakers, according to South Korea’s Defense Ministry.

South Korea raised its military readiness to its highest level.

Joint Chiefs of Staff spokesman Jeon Ha-kyu told a televised news conference that South Korea is ready to repel any additional provocation. Defense officials said South Korea will continue loudspeaker broadcasts despite the North Korean threats.

North Korea, which has also restarted its own propaganda broadcasts, is extremely sensitive to any criticism of the government run by leader Kim Jong Un, whose family has ruled since the North was founded in 1948. Pyongyang worries that the broadcasts could weaken Kim’s grip on absolute power, analysts say.

The artillery exchange also comes during another point of tensions between the Koreas: annual US-South Korean military drills that North Korea calls an invasion rehearsal. Seoul and Washington say the drills are defensive in nature.

South Korean President Park Geun-hye convened an emergency National Security Council meeting and ordered South Korea’s military to “resolutely” deal with any provocation by NorthKorea.

About 80 residents in the South Korean town where the shell fell, Yeoncheon, were evacuated to underground bunkers, and authorities urged other residents to evacuate, a Yeoncheon official told the Associated Press.

In the nearby border city of Paju, residents were asked to stay home. On Ganghwa Island, residents in villages near a site where South Korea operates one of its loudspeakers were also evacuated, according to island officials.

South Korea’s Yonhap news agency reported that a total of about 2,000 residents along the border were evacuated.

Pyongyang has claimed that Seoul fabricated its evidence on the land mines and demanded video proof.

While the Koreas regularly exchange hostile rhetoric, it is also not unusual for fighting to occasionally erupt. Last October, North Korean troops opened fire at areas in Yeoncheon, after South Korean activists launched balloons there that carried propaganda leaflets across the border. South Korea returned fire, but no casualties were reported. Later in October, border guards from the two Koreas again exchanged gunfire along the border, without any casualties.

Before that, the Koreas tangled in a deadly artillery exchange in 2010, when North Korean artillery strikes on a South Korean border island killed four South Koreans. Earlier in 2010, an alleged North Korean torpedo attack killed 46 South Korean sailors.

North Korea’s army said recently in a statement that the South Korean propaganda broadcasts were a declaration of war and that if they were not immediately stopped “an all-out military action of justice” would ensue.

South Korea has said the two soldiers wounded from the mine explosions were on a routine patrol in the southern part of the DMZ that separates the two Koreas. One soldier lost both legs and the other one leg.

The Koreas’ mine-strewn 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.