"No vehicles or trailers are readily visible anymore, which suggests that whatever was underway over the past four days is either done or the nature of it has changed. If the activity was test related, it would suggest that preparations are complete and that a test could occur at any time," 38 North said.

"However, based on satellite imagery alone, great caution should be used in making this determination. North Korea clearly understands that the world is watching and is undoubtedly, at some level, attempting to manipulate reporting about developments at Punggye-ri," it said.

On Wednesday, some 70-100 people were seen gathered at the site's main administrative area, an unusual scene last observed ahead of the North's fourth nuclear test in 2013. Other signs of test preparations included the laying of cables used in initiating a blast and collecting data from the explosion.

"The kind of 'signaling' we have seen over the past few days was not observed before either of the nuclear tests in 2016.

Similar activity was observed before the 2013 test, but that was before Pyongyang fully understood what we were looking for," 38 North said.

Satellite imagery alone cannot provide a precise answer as to when a new test will be conducted, it said. (Yonhap)