A North Korean fishing vessel was sent back home after it accidentally crossed the inter-Korean sea border early Wednesday, South Korea's military said.

The vessel was reported by a South Korean fishing boat around 3:40 a.m. some 21 kilometers south of the Northern Limit Line, the de facto maritime border, in the East Sea, according to the Joint Chiefs of Staff.

"A Navy ship was dispatched to the scene to search the boat, and found it had a problem with its navigation and (the crew) had no intention to defect to South Korea," the JCS said.

After verification, the North Korean boat and its crew were expelled to the North Korean side at around 7:10 a.m., it said.

"Our military is maintaining a high level of combat readiness and keeping close tabs on North Korean activities,” the JCS said.

In late May, two North Korean vessels had crossed the NLL in the Yellow Sea in what South Korean officials said was a test of Seoul's military's defense posture there.

At that time, the Navy fired five warning shots to notify them they were trespassing and to send them back north of the NLL.

North Korea occasionally sends its vessels south of the NLL, which they do not accept as an official sea border since the demarcation line was drawn up by the United Nations Command after the 1950-53 Korean War. (Yonhap)