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.
Pyongyang has conducted a blitz of weapons tests, including what it claimed were tactical nuclear drills, in recent weeks, with Seoul and Washington ramping up joint military exercises in response.
With talks long-stalled, North-South relations are at one of their testiest points in years, with Pyongyang's Kim Jong Un last month declaring his country an "irreversible" nuclear power, effectively ending negotiations over his banned weapons programs.
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 am (1842 GMT) 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".
According to AFP, Pyongyang's Korean People's Army said a South Korean military vessel had "invaded" the de facto border by 2.5 to 5 kilometers (1.5 to 3 miles) 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 spokesman 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.
