Some 10 buoys that North Korea installed last year to apparently identify the western inter-Korean sea border were washed away, a government source said Sunday, amid constant concerns that the communist state would violate the maritime boundary.

There are no signs yet of the North trying to reinstall the buoys -- with a diameter of less than 1 meter each - after it put in place a monitoring device on a deserted island some 12 kilometers northeast of the South Korean border island of Yeonpyeong, the source said.

"Around 10 buoys the North placed around the Northern Limit Line in the West Sea were swept away late last year," the source, who requested anonymity, said, referring to the de facto maritime border. "But it is not moving yet to put them there again."

Last May through June, the North put the buoys around the NLL, a flashpoint where three bloody naval skirmishes took place in 1999, 2002 and 2009.

The exact reason why the North deployed the buoys remains unknown. But military authorities believe the North that did not have a high-tech naval monitoring system might have used them to identify the NLL.

But late last year, the North built a 20-meter-high steel tower on Ari Island, an uninhabited island adjacent to Yeonpyeong Island and put a high-performance monitoring device on the tower. It also deployed some 30 personnel in charge of operating a radar system there.

The new monitoring equipment might be the reason why the North did not try to reinstall the buoys near the sea border, a military official said.

"As the North has installed the new device that can keep close tabs on the movement along the NLL in the West Sea, it might have thought that there is no need to put the buoys again there," he said on condition of anonymity.

In recent months, South Korean authorities have been reinforcing their monitoring activities near the border due to a spike in the number of North Korean and Chinese fishing boats catching crabs near the NLL. A daily average of around 380 boats including 140 North Korean vessels have been seen operating near the border, officials said.

Military tensions have run high along the sea border as Pyongyang has persistently disputed the border on the grounds that it was drawn unilaterally by the then U.N.-led United Nations Command after the end of the 1950-53 Korean War.

The North has demanded that the NLL should be redrawn further south. (Yonhap)