The report by the Institute for Science and International Security (ISIS) chimes with other findings and suggests Pyongyang is moving towards an underground detonation.
Pictures taken yesterday -- the first day of US President Barack Obama's visit to neighbouring South Korea -- show an increase in movement near one entrance to a tunnel.
"The images show in particular activity at the South Portal of the site, a possible site for North Korea's next nuclear test, and in the main support area," ISIS said in a release.
This was "probably related to preparations for a detonation," the institute said on its closely followed 38 North website.
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ISIS said higher resolution imagery, taken on Friday confirmed that analysis and suggested the preparations were continuing.
"On April 23, several containers were located in front of one of the South Portal's tunnel entrances," ISIS said.
"The higher resolution April 25 imagery shows more clearly what appears to be this collection of boxes or containers near this tunnel entrance."
However, the regime is notoriously unpredictable and observers warn that it is possible the preparations are a feint for the benefit of the satellites Pyongyang knows are watching.
The North has a long record of making threats in an effort to extract aid or concessions from the international community.
But activity at the site is being watched particularly carefully because of Obama's presence in the region amid speculation Pyongyang would like to thumb its nose at the US.
Politics would certainly play a role in the North's calculations, and it would be weighing the possible impact of detonation while Obama was in Asia.