Photos taken of the Punggye-ri nuclear test site appear to show the arrival of new equipment and a mining cart.
The secret pictures come amid reports the Hermit Kingdom is planning a nuclear bomb as it tried to develop a nuclear-tipped missile capable of hitting the US.
It is feared the blast will take place on Saturday, September 9, exactly a year after its fifth and last nuke explosion.
And because it coincides with a North Korean national holiday – Day of the Foundation of the Republic – it is believed the next test will take place on this day.
The new satellite images, taken on Sunday August 27, have been revealed by the 38 North website – a think-tank that monitors North Korea.
It states the images show “it (North Korea) could conduct a test on short notice”.
Such a test would be carried out underground, beneath Mount Mantap, which means signs of excavation are now being closely watched for.
An article on the 38 North website reads: “On Monday, August 28, 2017, media reports began surfacing which claimed that South Korea’s National Intelligence Service (NIS) had informed South Korean officials (in a closed-door session) that the NIS detected indications of preparations for another nuclear test at the DPRK’s Punggye-ri underground nuclear test site.
“One report more specifically stated that another test could fall on September 9, which is both a North Korean national holiday (the Day of the Foundation of the Republic) and coincident with the date chosen by North Korea to conduct it most recent, and fifth, nuclear test last year.
“However, commercial satellite imagery from August 27, 2017 does not provide observable corroborative evidence that the DPRK is about to conduct another underground nuclear test immediately. The situation is reminiscent of when we responded to similar reports in mid-June that a nuclear test was imminent at that time.
“Nonetheless, we remain firm in our previous assessments that the DPRK has, since April 2017, continued to maintain the site at a high state of readiness such that it could conduct a test on short notice, whenever the political decision is made to proceed with another test or tests.
“It is just that we cannot report that there is any observable evidence on the most recent commercial satellite imagery to show that such a decision has already been made.”
The national holiday next weekend – also known as Independence day – sees North Koreans celebrate 69 years since Kim Il-sung came to power.
It comes after Kim Jong-un’s army launched a ballistic missile over Japan that blew up in the Pacific Ocean last Friday night, sparking mass evacuations.