South Korea's defense ministry on Tuesday expressed "grave concerns" over North Korea escalating border tensions with threats and its criticism of South Korea's military operations conducted in response to the North's repeated border intrusions violating the armistice agreement, Yonhap reported.

The ministry's statement came a day after Kim Yo-jong, the powerful sister of North Korean leader Kim Jong-un, threatened to resume propaganda loudspeaker broadcasts against Seoul and deploy more tactical strike assets along the Demilitarized Zone (DMZ), saying the border situation has become "very unstable."

In a commentary released by state media Monday, she cited in specific numbers the warning shots and broadcasts from the South, claiming that those operations are intended to hamper its border fortification work.

"The Ministry of National Defense expresses grave concern over Kim Yo-jong's criticism of our military's legitimate operations conducted under the Armistice Agreement, as well as her escalation of tensions through military threats," it said in a statement.

"The warning broadcasts and warning shots conducted by our military near the Military Demarcation Line (MDL) are legitimate and defensive measures intended to prevent North Korean troops from violating the MDL and to repel those that cross it," the ministry said.

South Korea will "retaliate decisively" if the North engages in further provocations using the "legitimate" measures taken by Seoul as a pretext, the ministry warned, vowing consistent responses in accordance with the principles of the Armistice Agreement that ended the 1950-53 Korean War.

The ministry also urged Pyongyang to cease its far-fetched claims and incursions into the MDL and respond to Seoul's calls for efforts to resolve issues related to the border lines.

Earlier,  North Korea said it had conducted an underwater test of a new AI-powered high-speed precision-guided torpedo weapon system.

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