The U.S. urged North Korea to cease escalating tension on Wednesday after the communist country was found to have been setting up military bunkers on a border island in the scuffle-prone West Sea.

The facilities on Galdo Island, some 2.5 kilometers north of the Northern Limit Line, the de-facto naval frontier, are thought to be home to 122-millimeter multiple rocket launchers or guard posts to track the movement of South Korean marines and patrol boats.

Seoul’s Defense Ministry criticized the move on Tuesday, saying it poses a “grave threat” in light of the island’s geographic location and the proximity to the NLL and Yeonpyeongdo Island, about 4.5 kilometers from Galdo Island. Pyongyang attacked Yeonpyeongdo Island in November 2010, killing four South Koreans.

“We call on North Korea to refrain from actions that further raise tensions in the region and to focus instead on taking concrete steps toward fulfilling its international commitments and obligations,” a spokesperson at the U.S. State Department was quoted as saying by Voice of America.

“The United States remains steadfast in its commitments to defense and its allies. We will continue to coordinate closely with the Republic of Korea (South Korea).”

Tension has flared across the border in recent weeks as the Kim Jong-un regime conducts an underwater test of a submarine-launched ballistic missile, threatens to attack South Korean Navy ships in the West Sea and claims to be “miniaturizing and diversifying” means of a nuclear strike.

The chief nuclear negotiators of South Korea, the U.S. and Japan gathered in Seoul on Wednesday and hinted at imposing further sanctions and other measures to ramp up pressure on the unruly regime.

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