North Korea is building “several” military bunkers on a border island in the skirmish-prone West Sea that pose a “grave threat” to South Korea, Seoul’s Defense Ministry said Tuesday.

The facilities on Galdo Island, some 2.5 kilometers north of the Northern Limit Line, the de-facto maritime frontier, are expected to house 122-millimeter multiple rocket launchers or guard posts to monitor the movement of South Korean marines and patrol vessels.

With a range of 20 kilometers, the weapons were mobilized when Pyongyang bombarded Yeonpyeongdo Island ― merely 4.5 kilometers away from Galdo Island ― in November 2010, killing two Marines and two residents while injuring more than a dozen. 

The communist country embarked on excavation activities in the area in March and has since constructed at least five camps, news reports suggested, citing unnamed military authorities here.

“The North Korean military is establishing several covered bunkers on Galdo Island, north of Yeonpyeongdo Island in the West Sea,” ministry spokesperson Kim Min-seok said at a regular news briefing.

“Given Galdo Island’s geographic location and the distance between the NLL and Yeonpyeongdo Island, the activities present a grave threat to our military’s operations. We are intensively monitoring any deployment of firearms by the North Korean military.”

If the regime presses ahead to station an artillery battery, Galdo Island will be its nearest base to attack the South Korean border islands, which could also make it easier to target warships sailing nearby.

The closest existing base is Jangjaedo Island, around 7 kilometers away from Yeonpyeongdo Island.

The South Korean military has deployed Spike missiles which have a range of about 20 kilometers on Yeonpyeongdo and Baengnyeongdo islands to help defend the region.

The discovery came as the Kim Jong-un regime has been ratcheting up tension in recent weeks by testing a submarine-launched ballistic missile, threatening to attack South Korean Navy ships in the West Sea and claiming to be “miniaturizing and diversifying” means of a nuclear strike.

With South Korea and the U.S. exploring ways to stifle its behavior, including extra sanctions, concerns are mounting over a possible provocation such as in the West Sea. The North also staged night gunfire drills early this month.

The waters saw bloody cross-border clashes in 1999, 2002 and 2009. Months before the island shelling, the regime torpedoed a South Korean corvette near the NLL, claiming 46 lives.

By Shin Hyon-hee (heeshin@heraldcorp.com)