North Korea has covered the entrance to one of the tunnels at its underground nuclear site in an apparent effort to avoid satellite monitoring as it makes final preparations for an imminent nuclear test, South Korean news agency Yonhap reported Friday, citing intelligence sources.

“Analysis showed a camouflage net looking like a roof was placed on the tunnel entrance,” said an anonymous source. “The move seems to be aimed at keeping nuclear test preparations near their completion from being exposed outside.”

Another source, who also requested anonymity, told Yonhap that the cover may be aimed at confusing outside watchers before