North and South Korea on Thursday exchanged artillery fire along their heavily fortified border. The incident marked an apparent escalation of a dispute over the South's propaganda broadcasts along the border, which Pyongyang insisted must stop within 48 hours.

To underline its demand, the North launched shells, a provocation to which Seoul reacted by firing artillery rounds, Reuters reported. The North Korean regime warned Seoul in a letter that it would take military action if the loudspeaker broadcasts did not end, according to the South Korean defense ministry.

In a separate missive, however, Pyongyang said it was open to negotiations, even though it considers the propaganda a "declaration of war," South Korea's Unification Ministry added. Meanwhile, North Korea on Monday expanded its own propaganda broadcasts over the border.

"Our military has stepped up monitoring and is closely watching North Korean military movements," the South Korean defense ministry noted in a statement, according to NBC News.

The South's army "raised its vigilance to the highest level and is closely watching the movement of the North Korean military, ready to respond strongly and decisively to any further provocations from the North," Col. Jeon Ha-gyu, a military spokesman, told The New York Times.

The United States is keeping a close eye on the tensions, Pentagon spokesman Cmdr. William Urban told CNN. Thursday's incident was the first time the two Koreas had traded fire since October 2014, the news channel detailed.

The exchange did not result in any damage or casualties, but about 220 people from nearby villages were evacuated into underground shelters, said Hong Seong-beom, an official from Yeoncheon County in South Korea's western Gyeonggi Province, according to the New York Times.

In Seoul, President Park Geun-hye on Thursday convened an emergency meeting of the National Security Council and ordered the military to "deal resolutely with any North Korean provocations."

Koh Yu-hwan, a North Korea expert at Dongguk University, said he did not expect Pyongyang to escalate the exchange of fire into a major skirmish.

"The fact that the North only launched one rocket without hitting any South Korean loudspeaker indicates that the North meant it as a warning," he said.

The incident stemmed from an incident on Monday, Aug. 11 in which two South Korean soldiers were killed by "box" mines. South Korea accused its northern rival of planting them across the DMZ, prompting them to resume using the propaganda speakers.