South Korea's military said Tuesday it remains firmly committed to the defense of the Northern Limit Line, the de facto sea border with North Korea.
The two Koreas have long disputed the demarcation of their Yellow Sea border, a legacy of the 1950-53 Korean War, which ended in a truce.
The South has maintained the NLL, while the North has drawn its own "security line" further south, leading to some deadly naval skirmishes.
In April, the leaders of the two sides agreed to turn the tense area into a "maritime peace zone." They used the term "NLL" in their joint statement.
According to President Moon Jae-in, North Korean leader Kim Jong-un has recognized the NLL.
Concern, however, has been rekindled here, after the North resumed its assertion last weekend that South Korean ships illegally crossed its security line.
The North delivered a warning message twice to the South via a naval communication channel on Sunday, Seoul's defense authorities confirmed.
"Whatever North Korea's claim is, the NLL in the Yellow Sea is the actual maritime demarcation line that our troops have defended with blood," Army Col. Roh Jae-cheon, spokesman for the Joint Chiefs of Staff, said at a press briefing.
The military will continue to defend it absolutely, he added. (Yonhap)