South Korea's Coast Guard failed to detect a North Korean boat that entered South Korean waters last week despite the presence of patrol boats operating at the time. 

The small wooden boat carrying four North Koreans was reported by a civilian at the east coast port of Samcheok, Gangwon Province at 6:50 a.m. last Saturday. 

South Korea's Coast Guard said on Wednesday that two 50-ton coast guard ships were patrolling in waters near the port at the time, but failed to detect the North Korean vessel, adding the wooden boat was out of the patrol crafts' detection range.

The chief of the coast guard said he feels responsible and vowed to take proper personnel measures with those in charge of local coast guard units.

Video surveillance cameras operated by the coast guard reportedly picked up the North Korean ship at 6:23 a.m., but as there is apparently no official in charge of monitoring the feed, the footage was not discovered until about two hours later, well after a South Korean civilian first reported the boat.