A North Korean man, who was captured after crossing into South Korea, appears to have crossed the border via the sea while wearing a diving suit and a set of fins, the military said Wednesday.

On Tuesday, the man was spotted moving south in the early hours inside a restricted area north of the Civilian Control Line in the eastern border town of Goseong before being captured following a three-hour manhunt.

According to the Joint Chiefs of Staff (JCS), he is likely to have swum across the maritime border to arrive near an observatory south of South Korean general outposts (GOPs) and passed through a drainage conduit to evade fences set up along the shore.

The man was captured on the military's surveillance equipment multiple times after coming ashore but troops failed to take appropriate action and the drainage tunnel was also poorly equipped to prevent penetrations, JCS said.

"We take this situation gravely and an onsite probe is under way with the ground forces command," the JCS said in a statement. "Depending on the probe results, we will come up with follow-up measures and take stern actions."