A new structure seen inside the tunnel's entry on February 11, 2021. Photo: Maxar
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," South Korea's deputy ministry spokesman Col. Moon Hong-sik said during a regular briefing.

The ministry said last month that 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.