North Korea has carried out significant tunneling on the west side of its main nuclear test site, indicating the potential for another bomb detonation, a U.S. website said Thursday.
38 North, which monitors the reclusive state, said recent satellite imagery of the Punggye-ri nuclear test site shows increased activity around the West Portal through December last year.
"The North Portal, where the last five nuclear tests were conducted, remains dormant," it said, but "tunnel excavation has been stepped up at the West Portal."
Mining carts and personnel were consistently present there, it said, and there was "significant expansion" of the spoil pile.
The site also said that on Dec. 28, up to 120 personnel could be seen in seven different formations in the Southern Support Area, but their purpose is unknown.
"These activities underscore North Korea's continued efforts to maintain the Punggye-ri site's potential for future nuclear testing," the site said.
North Korea conducted its sixth and latest nuclear test in September, claiming it detonated a hydrogen bomb. (Yonhap)