The U.S.-based Web site 38 North reports that significant tunneling excavation is under way at North Korea's Punggye-ri nuclear test site, suggesting the area is being prepared for a future nuclear test.

The North Korea monitoring site said on Thursday that throughout December 2017, there were "mining carts and personnel" as well as what appeared to be a "spoil pile" that had been greatly expanded at the test facility's west portal.

The report is based on an analysis of new commercial satellite images of the nuclear site by Frank Pabian and two other nuclear experts. 

It said that on December 28th there were a large number of personnel, roughly 100 to 200, observed in seven different formations whose purpose is unknown in the southern support area.

The report concluded that these activities underscore North Korea’s continued efforts to maintain the Punggye-ri site’s potential for future nuclear testing.