A North Korean vessel thought to be a military patrol ship violated the western inter-Korean sea border and retreated back to its waters Tuesday night after a South Korean warship fired warning shots, the Joint Chiefs of Staff said Wednesday.

“At 9:38 p.m. Tuesday, an unidentified North Korean ship invaded our waters and continued to 700 meters south of the Northern Limit Line (sea border). Our patrol ship warned the ship three times and fired eight warning shots at it,” said a JCS official, declining to be named.

“The ship remained in the South Korean waters until 9:47 p.m. The warning shots were fired from 9:42-43 p.m. The rounds landed some 500 meters away from the North Korean ship.”

Should the ship be confirmed as a military one, it will have been the first NLL violation this year by the North Korean military, officials said. Fishing boats from the North have violated the border several times this year.

There was no word on the NLL violation from the communist state yet, the JCS official said. 

At the time of the violation, some 100 Chinese fishing boats were spotted north of the NLL and there were no South Korean vessels near the restricted area, he said.

North Korea has challenged the NLL in a move to make the South’s frontline areas around the border a disputed sea. The North insists that the border should be pushed further south as it was unilaterally drawn by the U.S.-led U.N. Command at the end of the 1950-53 Korean War.

The area near the western sea border has been regarded as a powder keg. It was the scene of deadly inter-Korean naval skirmishes in 1999, 2002 and 2009. In March last year, the corvette Cheonan sank in a North Korean torpedo attack.

By Song Sang-ho (sshluck@heraldcorp.com)