Fresh satellite images indicate that activity has resumed at North Korea's nuclear test site, U.S. analysts said Tuesday, amid fears the reclusive nation may soon conduct its sixth atomic test.

In a report by the 38 North website, images of the Punggye-ri nuclear test facility taken April 25 showed that the pumping of water from the site's north portal — where the North has apparently been preparing to conduct an atomic test — had been restarted "to maintain an optimal environment for instrumentation and stemming."

Removing the water from the tunnel is presumably done to keep it dry for monitoring or communications equipment.