North Korea has set up a "peacetime firing zone" along its maritime border with South Korea, Seoul's Yonhap agency reported.
The buildup of tensions in the Yellow Sea follows Pyongyang's accusations last week of South Korean "provocations" near the border, after the South Korean Navy carried out a series of test explosions during drills.
The navies clashed last month on a disputed area of the Yellow Sea border.
A North Korean patrol vessel entered the area, prompting a South Korean border ship to fire warning shots. The North fired back, but no one was injured.
In 1999 and 2002 deadly clashes occurred along the disputed demarcation line, unilaterally drawn by the United States in 1953 and not recognized by North Korea.
Nonlethal armed incidents have also occasionally taken place along the border, which is called the Northern Limit Line.
MOSCOW, December 21 (RIA Novosti)