Satellite imagery of North Korea's main nuclear test site "strongly suggests" that preparations for the country's sixth atomic test are entering the final stages, a leading U.S. website that monitors the isolated nation said Tuesday.

The 38 North website, a project of the U.S.-Korea Institute at Johns Hopkins University, said imagery taken Saturday of the Punggye-ri nuclear test site shows the continued presence of three to four vehicles and trailers at the entrance to an underground test tunnel.

According to the website, the texture of the ground from the tunnel entrance past the vehicles and trailers "suggests that communications cables may have been laid on the ground." This equipment would likely be used to initiate the test and then collect and process data from the explosion. The report said water was also being pumped out of the portal or being drained downhill, "presumably to keep the tunnel dry for monitoring or communications equipment."