The North sent a warning message to South Korea's presidential office for the second straight day on Saturday, claiming that Southern speedboats had intruded its Yellow Sea border "frequently" in the recent past.
The poorly marked inter-Korean border in the Yellow Sea has been the site of several bloody skirmishes between the navies of the two countries in the past, resulting in dozens of soldiers killed or wounded on both sides.
The North does not recognize the Yellow Sea border, commonly called the Northern Limit Line (NLL) which was drawn unilaterally by the U.S.-led U.N. forces at the end of the 1950-53 Korean War, which ended in a cease-fire, not a permanent peace treaty.
The North has long insisted that the line be drawn farther south, a demand rejected by South Korea.
Adm. Choi Yun-hee, the chairman of South Korea's Joint Chiefs of Staff, visited the Navy's western command headquarters on Saturday, vowing to defend the Yellow Sea border against possible provocations from North Korea.
Also on Saturday, North Korea test-fired three anti-ship missiles into the sea off its east coast in what was seen as its latest show of force against Seoul.
South Korean military officials identified the North's anti-ship missiles KN-01 cruise missiles and said the missiles were fired off into the sea off Wonsan, a major port on the North's east coast, in a span of one hour starting at 4:25 p.m.
The missiles with a range of 100 kilometers are believed to have been modified from Chinese Silkworm missiles, they said.
The North's Korean Central News Agency also reported that leader Kim Jong-un observed an underwater test-fire of a submarine-launched ballistic missile. It did not specify the date and location of what could be the country's first test-fire of a ballistic missile from a submarine.
South Korean officials said the North's submarine-launched missile was more of a test for an ejection rather than firing, since the missile is assumed to have flown only about 100 meters after it soared from the waters. (Yonhap)