A small wooden boat carrying three North Koreans crossed the inter-Korean maritime border in the East Sea and was towed into a South Korean port for investigation.

South Korea’s Joint Chiefs of Staff (JCS) said on Sunday that the boat was spotted crossing the Northern Limit Line (NLL), the de facto maritime border between the two Koreas, at around 11:21 p.m. Saturday, prompting the military to immediately send a naval vessel to the scene.

The three crew members and the boat were then brought to a South Korean military port in Yangyang, Gangwon Province, at around 5:30 a.m. Sunday for investigation.

It is unusual for the South Korean military to tow North Korean ships crossing the maritime border into the South, as it usually sends them back home right away.

According to a JCS official, the crewmen claimed that their boat had crossed the NLL as they lost direction, and cited "navigation error." But the military took them to the port to confirm the exact reason for the NLL crossing.

The JCS added that the wooden ship was found in waters where no other fishing boats were operating and directly headed towards the South at a consistent speed.

The JCS official also said that while one crew member was clad in a military uniform, it remains unconfirmed whether he is anactive-duty soldier.