North Korea on Thursday threatened military action against South Korea if the country continued its propaganda warfare, Yonhap, South Korea’s official news agency, reported. The threat comes hours after the two countries exchanged shells over the heavily fortified border near the South’s western province of Gyeonggi.

Seoul fired dozens of 155 millimeter shells after its military radar system found that the North was firing at a South Korean front-line military unit in Yeoncheon city, Yonhap, South Korea’s official news agency, reported. Following the attack, South Korean President Park Geun-hye presided over a National Security Council session, while the country’s military was put on “high readiness,” according to Yonhap.

South Korean residents in the border area were also evacuated and no immediate damage was reported.

The latest firing between the two countries follows accusations from the South that Pyongyang detonated landmines in the demilitarized zone near the border, injuring two of its soldiers on Aug. 4. North Korea denied the allegations, calling them fabricated, but Seoul promised a “harsh” response to the attack. On Thursday, the South also sent a letter to the United Nations about the detonation of the landmines, Yonhap reported, citing a foreign ministry official.

Tensions between the two Koreas further escalated after Seoul began broadcasting propaganda messages over the border, to which the North responded over the weekend by threatening to blow up speakers. In retaliation, Pyongyang also began its own propaganda broadcasts.

The situation worsened after Seoul started the joint annual Ulchi Freedom Guardian drills with the U.S. on Monday, aimed at prepping its defense in case of an attack from the North. Pyongyang termed the move as a “declaration of war” and threatened to strike the White House and Seoul, unless the drills were stopped.