North Korea has covered the entrance to one of the tunnels at an underground nuclear site in an apparent effort to avoid satellite monitoring as the communist nation makes final preparations for an imminent nuclear test, intelligence sources said Friday.

The latest move comes as South Korea and the United States are mobilizing intelligence assets, including spy satellites, to detect early signs of a third atomic test in Punggye-ri in the North's northeastern tip. The sprawling nuclear test site in mountainous terrain has three known tunnel entrances and multiple support buildings.

"Analysis showed a camouflage net looking like a roof was placed on the tunnel entrance," a source said, requesting anonymity, as he is not allowed to discuss military information.

"The move seems to be aimed at keeping nuclear test preparations near their completion from being exposed outside."

Another source, who also requested anonymity due to sensitivity of the issue, said the cover may be aimed at confusing outside watchers before detonating the nuclear device.

"It seems like a disturbing tactic, similar to one that was used when the North prepared for a long-range rocket in December last year," the source said.

Days before the Dec. 12 rocket launch, Pyongyang placed a camouflage net on a launch pad in its northwestern tip and assembled the three-stage rocket with the cover on, a move interpreted as evading spy satellites.

Citing satellite imagery that shows increased activity near the site, Seoul officials have been placed on high alert to cope with a third nuclear test, which could further escalate tensions in the region.

"North Korea has come to a level to be able to detonate a nuclear device any time if the leadership makes a decision," Wi Yong-seop, a defense ministry official said in a briefing. "Forces of South Korea and the U.S. are closely monitoring North Korea's preparations."

Pyongyang detonated nuclear devices at the Punggye-ri test site in 2006 and 2009, following long-range rocket launches. (Yonhap News)