Satellite imagery of the Punggye-ri nuclear test site in North Korea on Mar. 25, 2017. DigitalGlobe | 38 North | Getty Images

Satellite imagery of North Korea's main nuclear test site taken over the weekend indicates that Pyongyang could be in the final stages of preparations for a sixth nuclear test, a U.S. think tank reported on Tuesday.

Washington-based 38 North, a website that monitors North Korea, said the images from Saturday showed the continued presence of vehicles and trailers at the Punggye-ri test site and signs that communications cables may have been laid to a test tunnel.

Water was also being pumped out and was draining downhill "presumably to keep the tunnel dry for monitoring or communications equipment," it said.

"The combination of these factors strongly suggests that test preparations are well under way, including the installation of instrumentation. The imagery, however, does not provide any definitive evidence of either a nuclear device or the timing of a test."



A lack of activity elsewhere at the site "may mean that test preparations are in their final stages," the report said, although it added: "Since North Korea knows the world is watching and is capable of deception, caution should be used before declaring that a nuclear test is imminent."

The think tank reported over the weekend that the vehicles and trailers at the site could have been used to install a nuclear device for an underground test.

Another 38 North report said recent satellite images of North Korea's nuclear research center at Yongbyon showed rail and vehicle movements that could indicate a plan to resume reprocessing of spent fuel rods to produce plutonium for bomb fuel.

North Korea has conducted five nuclear tests and a series of missile launches, in defiance of United Nations sanctions, and is believed by experts and government officials to be working to develop nuclear-tipped missiles that could reach the United States.

