Two North Korean men swam across the Yellow Sea border to defect to South Korea, government sources here said Thursday.

South Korean troops spotted them reaching Gyodong Island, just south of the Northern Limit Line (NLL), at around 4 a.m. Thursday, according to the sources. The island is about 2.5 kilometers away from the North's closest western coast.

The two, one in his 50s and the other in his 20s, immediately expressed their desire to defect to the South.

They were taken into custody for interrogation by the South's state intelligence authorities on their reasons for defection, said the sources.

It's quite unusual for North Koreans to swim across the heavily-fortified border to reach South Korea.

They usually cross the porous border with China first and then try to head to South Korea, often through Southeast Asian nations.

In August, a North Korean man defected to the South on a wooden boat. (Yonhap)