South Korea and the United States are closely following movements at North Korea's nuclear facilities, the defense ministry said Thursday, amid a report of new structures at one of the sites.

On Tuesday, CNN reported that the North may have taken steps to hide a facility at the Yongdoktong site that the US believes is used to store nuclear weapons, citing recent satellite images that show new structures.

"The intelligence authorities of South Korea and the US are closely monitoring activities related to North Korea's nuclear facilities," deputy ministry spokesman Col. Moon Hong-sik said during a regular briefing, without further elaboration.

The ministry said last month North Korea has been expanding missile development facilities but has shown no unusual signs at its demolished Punggyeri nuclear testing site or the long-suspended nuclear reactor at Yongbyon complex.

"Our government is putting utmost efforts on close cooperation with the international community to achieve the complete denuclearization of the Korean Peninsula," Moon said. (Yonhap)