A group of opposition lawmakers made a protest visit to the defense minister on Friday over a recent border security failure that led to the undetected arrival of a North Korean boat at an eastern sea port.

Six lawmakers from the main opposition Liberty Korea Party (LKP) met with Defense Minister Jeong Kyeong-doo and lodged a protest against the military's dubious handling of the June 15 arrival of the boat at the eastern port of Samcheok.

Criticism has mounted following revelations that the military was in the dark until the boat reached the shore, some 130 kilometers away from the eastern sea border. Fueling the criticism were allegations that the military gave an incorrect account of what happened in a suspected cover-up attempt.

"Despite such gigantic loopholes in our surveillance operations, the JCS has said there were no problems. ... We need to clarify what caused and who instructed such distortion," Rep. Kim Young-woo told reporters after arriving at the ministry.

He also called on the government to come clean on the exact identities of the four North Koreans aboard and the reasons behind the government's "hasty repatriation" of the two.

According to intelligence authorities, their initial probe showed that they were all civilians. Of the four, two were returned home according to their wishes days after they arrived here, with the remaining two staying here as they expressed their desire to defect to the South.

The conservative LKP has been urging Jeong to resign over the incident and has vowed to push for a motion for his dismissal.

In order to find what went wrong, a government panel has been investigating related military units and officers, over possible slack discipline and attempts to cover up the case. (Yonhap)