Seoul, Oct 19 : A North Korean soldier has been taken into custody after crossing the Military Demarcation Line (MDL) on Sunday, the South Korean military said.

The Joint Chiefs of Staff said it detected and tracked the individual near the MDL earlier in the day, with relevant authorities planning to conduct an investigation into the motive, Yonhap News Agency reported.

It marks the first presumed defection by a North Korean soldier since the Lee Jae Myung government came into power.

On September 26, a North Korean merchant vessel briefly crossed the Northern Limit Line (NLL) in the Yellow Sea, the de facto maritime border between the Koreas, on Friday, but retreated after South Korea's military fired warning shots, officials said.

The 140-metre-long North Korean vessel crossed the NLL near the South Korean border island of Baengnyeong at 5.06 a.m. and remained on the southern side for an hour, according to the Joint Chiefs of Staff (JCS).

The vessel, which departed from a port in North Korea, reportedly advanced up to 5 kilometres south of the NLL.

The South Korean military, which had been monitoring the North Korean ship's advance toward the NLL, issued warning broadcasts, but the vessel crossed the line, triggering a South Korean Navy vessel to fire around 60 warning shots and shells.

The North Korean ship subsequently veered westward and retreated from the South's maritime control area at around 6 a.m., JSC officials said.

The 2,800-ton ROKS Cheonan frigate was conducting operations in the area at the time of the NLL breach.

A JCS official said a dozen Chinese ships were situated near the NLL at the time, adding it was looking into the possibility that the North Korean vessel crossed the maritime border to avoid the Chinese ships nearby.

The North Korean ship, named the Toksong, reportedly changed the nationality in the ship's automatic identification system to China after crossing the NLL, in an apparent move to conceal its identity, the official said.

The ship also had a Chinese flag attached, according to the official.

/IANS