North Korea's navy has bolstered its coastal patrol to deter its nationals from defecting to South Korea via the sea, South Korean sources said Sunday.

"North Korean patrol boats have ratcheted up their monitoring in waters off Tungsangot, Haeju and Kangryong," one source said, referring to areas all located in South Hwanghae Province on the west coast. "These moves appear designed to keep North Koreans from defecting on boats and to inspect small ships suspected of attempting to defect."

On Oct. 30, 21 North Koreans, including eight children, defected aboard a five-ton wooden boat in the Yellow Sea, while another man crossed the maritime border on a raft on the same day.

A second source said in response to these defections, North Korean coastal guards and ground forces have been placed on heightened alert.

The source added that some North Korean boats that once guarded waters near the Northern Limit Line, a de facto maritime border, to check for illegal fishing by Chinese boats left the area to reinforce coastal patrolling.

"It's not that North Korea is trying not to provoke South Korea by pulling boats away from near the NLL," the source explained.

"North Korea is just putting more emphasis on preventing more defections."

Waters near the NLL have seen deadly naval skirmishes and armed provocations by North Korea. Pyongyang refuses to recognize the NLL, drawn by the U.N. forces at the end of the 1950-53 Korean War, and has demanded it be redrawn farther south.

Nearly 23,000 North Koreans have defected to the South since the Korean War, amid chronic food shortages and harsh political oppression. While most cross the land border with China to reach South Korea, some have recently taken the sea route. (Yonhap News)