SEOUL, South Korea (AFP) — South Korea's military said Monday it had fired warning shots at a North Korean ship that crossed the countries' de facto maritime border, prompting the North to fire a warning in return.

Never officially delineated by the 1953 armistice agreement that brought Korean War hostilities to a close, the maritime border remains a flashpoint and has been the location of several previous clashes between the two sides.

A North Korean merchant vessel crossed what is known as the Northern Limit Line at 3:42 a.m. but retreated north after Seoul's navy fired warning shots, South Korea's Joint Chiefs of Staff said.

"The North's continuing provocations and reckless claims undermine peace and stability on the Korean peninsula and international community," JCS said in a statement, urging Pyongyang to "stop immediately."

North Korea response

Pyongyang's Korean People's Army said a South Korean military vessel had "invaded" the de facto border by 2.5 to 5 kilometers a few minutes later and that the KPA fired 10 warning rounds from the country's west coast.

KPA "coastal defense units on the western front… took an initial countermeasure to powerfully expel the enemy warship by firing 10 shells of multiple rocket launchers toward the territorial waters, where naval enemy movement was detected, at 5:15," a KPA General Staff spokesperson said in a statement carried by state media.

"The KPA General Staff once again sends a grave warning to the enemies who made (a) naval intrusion in the wake of such provocations as the recent artillery firing and loudspeaker broadcasting," he said.

North Korea has fired multiple artillery barrages this month into a maritime "buffer-zone" that was set up in 2018 as a way to reduce tensions between the two countries during a period of ill-fated diplomacy.

The moves are part of a dramatic increase this year in what Seoul calls "provocations" by the North, including conducting its longest-ever missile launch by distance, which overflew Japan and prompted rare evacuation warnings.

Seoul has also recently conducted live-fire drills and the United States re-deployed a nuclear-powered aircraft carrier to the region to conduct large-scale trilateral drills also involving Tokyo.

"The merchant ship's incursion and the North's artillery firings demonstrate the lack of consensus on the NLL border," Cheong Seong-chang, a researcher at the Sejong Institute told AFP.