North Korea watchdog 38 North reports there has been a “consistently high” level of activity at the West portal of the Punggye-ri Nuclear Test Site.
Mining carts, vehicles and personnel have been seen at the site suggesting a tunnel excavation is underway, as Pyongyang readies the tunnel for further nuclear testing.
North Korean intelligence recently reported “construction had resumed at a fourth tunnel (West Portal), making it unable to be used ‘for a considerable amount of time.’”
And the images show no new spoil has been added to the site, but presence and movement of equipment has significantly increased.
The West tunnel complex has been unused as yet, and no activity has been spotted in the area over the past several months.
Five of the past six nuclear tests were conducted at the North Portal, but no vehicles or equipment have been seen there since September.
A recent report by TV Asahi claimed hundreds of North Korean personnel died after the tunnels collapsed at the test site.
The three recent tremors could have caused damage to the tunnel networks but satellite images cannot confirm this.
The images come as CNN reports Kim Jong-un could be developing bioweapons amid escalating tensions on the Korean peninsula.
Kim is said to have sent experts around the world to get degrees in microbiology and stepped up moves to build more factories and laboratories.
And the leader of the hermit nation has steadily been acquiring new machinery that could be used to churn out deadly microbes, US and Asian intelligence officials have claimed.
US analysts are concerned the research could mean the regime is producing dangerous microbes and pathogens that could be loaded into bioweapons.
Bioweapons can carry lethal viruses like smallpox and anthrax and are capable of killing hundreds of people and cause pandemics.
A US official told the Washington Post: “That the North Koreans have biological agents is known, by various means. The lingering question is, why have they acquired the materials and developed the science, but not yet produced weapons?
“If it started tomorrow we might not know it, unless we’re lucky enough to have an informant who happens to be in just the right place.”
Pyogyang’s most recent missle test, during the early hours of November 29, flew higher and farther than any other previous tests. The missile launch, which landed in Japan’s exclusive economic zone, came after a break of almost two months in testing.