(Yonhap photo)

SEOUL – South Korea's military said Monday it has fired warning shots at a North Korean ship that crossed the Northern Limit Line (NLL) in the Yellow Sea, the de facto maritime border.

North Korea also opened "warning fire" against South Korea in response.

A North Korean merchant vessel violated the NLL in waters near the front-line island of Baengnyeong at 3:42 a.m. and it retreated northwards after the South's Navy issued warning messages and fired some 20 rounds of warning shots, according to the South's Joint Chiefs of Staff (JCS).

Shortly after the announcement, the North Korean military claimed a South Korean warship violated the western sea boundary and it fired 10 artillery shells into the western sea from multiple rocket launchers in its "threatening and warning fires."

The South Korean escort ship "invaded" the Military Demarcation Line controlled by the North's military by 2.5-5 kilometers at 3:50 a.m. on the excuse of cracking down on an unidentified vessel, an unnamed spokesman for the General Staff of the Korean People's Army (KPA) said in a statement.

"The KPA General Staff once again sends a grave warning to the enemies who made even naval intrusion in the wake of such provocations as the recent artillery firing and loudspeaker broadcasting on the ground front," read the English-language statement carried by the Korean Central News Agency.

The South's JCS said it detected the North's launch of 10 artillery shells, which started at around 5:14 a.m., in violation of a 2018 inter-Korean military agreement aimed at reducing border tensions.

It also added multiple South Korean naval vessels, including a frigate, were deployed near the area, but they did not cross the NLL.

North Korea's ships have frequently intruded past the NLL, as it has long demanded that the line be moved farther south. The North does not recognize the border, arguing it was unilaterally drawn by the US-led UN Command after the 1950-53 Korean War. The two Koreas fought bloody battles there in 1999, 2002, and 2009.

The exchange of warning fire between the two Koreas came amid heightened tensions on the peninsula amid North Korea's continued provocations, including a barrage of ballistic missile tests and firing of artillery shots.

Pyongyang has carried out a string of weapons tests in recent weeks in response to joint military exercises between Seoul and Washington, which the North views as rehearsals for an invasion against it.

Speculation is rampant that North Korea may soon conduct a nuclear test as China's key party congress ended on the weekend.

On North Korea's assertion regarding loudspeaker campaigns, the South Korean military said it is no longer operating such propaganda broadcasts along the border. Recently, the South's military has used a similar broadcasting device installed at guard posts for the notification of choppers being mobilized for operations to put out wildfires or transport emergency patients, a defense official here said.

In 2018, the two Koreas dismantled around 40 loudspeakers each from their border regions, as they agreed to halt all hostile acts against each other under an agreement of the inter-Korean summit in April that year between then President Moon Jae-in and North Korean leader Kim Jong-un. (Yonhap)