A South Korean vessel tows a wooden boat that crossed the inter-Korean border to a port in the coastal county of Yangyang, 150 kilometers northeast of Seoul, on Oct. 24. Yonhap

A group of four unidentified individuals from North Korea crossed the eastern maritime inter-Korean border on a wooden boat early Tuesday, officials said, in an apparent attempt to defect to South Korea.

The Coast Guard and military officials secured the 7.5-meter-long boat carrying the North Koreans in waters east of Sokcho, 151 kilometers northeast of Seoul, a military official said, after South Korean fishermen at the scene reported seeing an "unusual" boat at around 7:10 a.m.

The group consists of one man and three women, according to Coast Guard officials. Details of their identities have not been made known.

Military, government and intelligence officials are expected to conduct a joint probe soon into their exact motive for crossing into South Korea.

A unification ministry official declined to confirm whether they have a "genuine" intent to defect to the South, noting an investigation is under way.

The latest incident raised questions over possible lapses in the South Korean military's maritime surveillance, as it did not initially detect the boat crossing the Northern Limit Line (NLL), a de facto maritime border, in the East Sea.

The military claimed it detected North Korean military activity above the eastern NLL early in the morning and dispatched naval assets, including maritime patrol aircraft, nearby.

Troops then spotted an "unusual" target in the waters using radar and thermal observation devices, and the military later confirmed it to be the boat reported by the fishermen, the military official said on condition of anonymity.

It marked the first time since November 2019 that a group of North Koreans has made an attempt to defect to South Korea on a vessel through the East Sea. At that time, South Korea repatriated the two North Koreans, who confessed to killing fellow crew members, back to their home country.

In May, the South Korean military intercepted a North Korean fishing boat carrying a group of defectors that crossed the western NLL in the Yellow Sea. (Yonhap)