North Korea has covered the entrance to a tunnel at its nuclear test site in an apparent effort to avoid satellite monitoring ahead of a widely expected detonation, a report said Friday.

The Punggye-ri site, which has three tunnel entrances and multiple support buildings, has been closely monitored by US, South Korean and Japanese intelligence since North Korea ramped up its test threats just over a week ago.

"Analysis showed a camouflage net looking like a roof was placed on the tunnel entrance," Yonhap News Agency cited a senior South Korean government source as saying.

"The move seems to be aimed at keeping nuclear test preparations -- now near their completion -- from being exposed outside," the source said.

North Korea has threatened to carry out its third nuclear test in response to UN sanctions imposed on Pyongyang for a long-range rocket launch it carried out in December.

The North said the launch was a scientific mission aimed at placing a satellite in orbit, but most of the world saw it as a disguised ballistic missile test.

Yonhap quoted an intelligence source as saying the cover at the test site may be aimed at confusing outside watchers.

"It seems like a disturbance tactic, similar to one that was used when the North prepared the rocket launch," the source said.

The South's intelligence agencies had been embarrassed by the timing of the December launch, partly due to the North's use of camouflage at the launch site.

At one point, diplomatic and military sources suggested the launch was facing a lengthy delay, while some said the rocket had been completely removed from the pad.

The rocket blasted off a day later.

The heightened tensions on the Korean peninsula come ahead of a series of military exercises by South Korea this month, including separate joint drills with US forces.

During an inspection Thursday of a US nuclear submarine that will be taking part in one drill, the chairman of South Korea's Joint Chiefs of Staff, Jung Seung-Jo, mentioned a "bustle of activity" detected at the Punggye-ri site.

"I can't discuss the details, but we are paying close attention to see whether this activity is a screen or preparation for an actual nuclear test," Yonhap quoted Jung as saying.