"Significant tunneling" excavation is underway at North Korea's Punggye-ri nuclear test site and shows the regime's continued efforts to maintain the site for potential future nuclear testing, a think tank specializing in tracking North Korean activities reported Thursday.

It follows reports in October that the test site is unstable and experienced tunnel collapses that have killed several hundred North Korean laborers.

The report on the 38 North website was based on an analysis of new commercial satellite images released of Punggye-ri, where the North Koreans have conducted the last six underground nuclear tests. It said 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 test site's north portal, used in the last five nuclear tests, "remains dormant," but there's new activity at the west portal, according to the 38 North, a think tank at the Johns Hopkins School of Advanced International Studies. Yet, it said there appears to be draining going on at the entrance to this portal.

The last nuclear test at Punggye-ri was conducted in September. Pyongyang claimed that blast was a miniaturized hydrogen weapon designed for an intercontinental ballistic missile.