The South Korean military has raised the alert status near the western sea border after a number of North Korean fishing boats were detected crossing the sea border near the crab-rich waters over the past week, a senior military official said Friday.

The latest move comes as a growing number of North Korean boats have been seen fishing in waters south of the Northern Limit Line, the de facto maritime boundary between the two Koreas.

Seven boats crossed the demarcation line twice on Sept. 12 and retreated after warning messages from South Korean patrol boats, and a group of two or three fishing boats repeatedly appeared near the NLL on Sept. 14, 15 and 20, the official said.

He did not give the total number of North Korean ships detected by the South Korean military.

"Our military is closely monitoring to figure out the intention of the North Korean boats' border crossings," the official said, asking anonymity due to the sensitivity of the issue. "If North Korean boats repeatedly cross the NLL for fishing, the military will promptly and sternly respond, without hesitation."

The North's military has not shown any special signs of provocation, the official added.

About 100 North Korean boats and 300 Chinese ships are currently fishing crabs in waters north of the NLL in the peak crab season, according to officials.

Another official questioned the North's intention of frequent border crossing at this time with the presidential elections only three months away.

"The North's NLL violations, four times this year alone, clearly seem to have an intention," the official said, adding that the North "may try to disturb South Korea by creating military tension ahead of (December's) presidential election."

The NLL has been the scene of several bloody skirmishes between the navies of the two Koreas. Most recently, tensions escalated after Pyongyang shelled the front-line island of Yeonpyeong near the border in November 2010, killing four South Koreans.

North Korea does not recognize the NLL, arguing it was unilaterally drawn by the U.S.-led United Nations forces at the conclusion of the 1950-53 Korean War that ended in a truce. It has demanded that a new border be drawn further south. (Yonhap)