South Korea's Defense Ministry said its navy fired warning shots today after three North Korean fishing boats crossed the nations' disputed Yellow Sea border. There were no immediate reports of damage or injuries.
The incident occurred near Yonpyong Island, west of the Korean peninsula.
The fishing boats turned back after South Korean navy ships fired shots into the air, a ministry spokesman said. It was the sixth encounter between North and South Korean vessels along the disputed sea border in seven days.
The maritime border between the two Koreas is not clearly marked, and North Korean fishing boats occasionally cross into South Korean waters during the crab catching season, which peaks this month.
South Korea is studying whether the repeated violations are intentional, a military spokesman said. It sent a protest letter to North Korea on Wednesday, urging Pyongyang to prevent such crossings.
On Thursday, North Korea accused the South Korean navy of repeatedly violating its territorial waters off the west coast and warned of "irrevocable serious consequences." South Korea's Defense Ministry dismissed the accusations.
A clash last June between the two sides in the same area left at least four South Korean sailors dead. In June 1999, an estimated 30 North Korean sailors died and seven South Koreans were injured in an exchange of gunfire.
In both incidents, North Korean naval vessels were reportedly sailing with a fleet of fishing boats. After last year's clash, the Pyongyang regime issued a statement expressing regret.
South Korea recognizes a Yellow Sea border demarcated by the U.N. North Korea claims a boundary farther south.