CAMP HUMPHREYS, South Korea — North and South Korea exchanged warning shots along their western sea border Monday, the same day Seoul and Washington kicked off four days of naval drills “in preparation for enemy provocations,” the South’s Ministry of National Defense said.

A North Korean merchant ship crossed the Northern Limit Line in the Yellow Sea, also known as the West Sea, that morning, according to the South’s Joint Chiefs of Staff. The South fired warning shots, to which North Korean forces responded by firing 10 artillery rounds into the water.

No injuries were reported by the South Korean military.

The North’s state-run Korean Central News Agency claimed in a report Monday it had issued a “grave warning” after a South Korean vessel crossed the maritime border.

The Northern Limit line is a de facto maritime boundary separating the two Koreas since the 1950-53 Korean War. The area has been marked with clashes in the past, including a 2010 incident in which 46 South Korean navy sailors were killed after their warship was hit by what international investigators concluded was a North Korean torpedo. Pyongyang denied involvement in the ship’s sinking.

The Sohae Joint Exercise, which involves military forces and law enforcement in the area, was planned prior to the incident, the Defense Ministry said.

The four-day drills include over 20 South Korean warships, F-15K Slam Eagles and KF-16 fighters, according to a ministry news release. The U.S. is sending AH-64 Apache helicopters and A-10 Thunderbolt II attack aircraft, it added.

The exercise is being held “in preparation for enemy provocations,” according to the release, and scenarios include detecting and intercepting high-speed hovercraft, infiltration of islands by special warfare units and countering “local provocation operations.”

“This is an exercise, which is defensive in nature, with an aim to improve capabilities to conduct joint and combined operations in preparation for a range of maritime provocations,” South Korean 2nd Fleet commander Adm. Ryu Hyosang said in the release. “We will defend our territorial waters by adamantly punishing enemies as we trained in preparation for enemy provocations.”

South Korean and U.S. forces are also scheduled to take part in Vigilant Storm, a five-day joint air exercise starting Oct. 31. Around 240 South Korean aircraft are expected to take part in the exercise along with about 100 U.S. aircraft, including helicopters and F-35A Lightning II stealth fighters.

The number of large-scale military exercises between Seoul and Washington has risen sharply in the past year. South Korean President Yoon Suk Yeol, a member of the conservative People Power Party who was inaugurated in May, has vowed to reinforce the country’s military alliance with the U.S. and sternly address North Korea’s provocations.

North Korea has denounced the allies’ military drills and has conducted a record of 26 rounds of missile tests so far this year. The communist regime is also suspected by U.S. and South Korean officials to have prepared to conduct its seventh nuclear test, its first since 2017.

David Choi is based in South Korea and reports on the U.S. military and foreign policy. He served in the U.S. Army and California Army National Guard. He graduated from the University of California, Los Angeles.

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