An analysis Monday by the 38 North website said commercial imagery taken last week showed several operational support buildings had been razed, and rails for mining carts apparently removed.
North Korea said it will dismantle its Punggye-ri test site between May 23 and 25 in the presence of local and international media.
The site was used for each of its six underground nuclear test explosions.
South Korea's president on Monday welcomed the announcement as the beginning of North Korea's nuclear disarmament. It comes ahead of a historic US-North Korea summit.
The analysis said no tunnel entrances at the test site appeared to have been permanently closed yet.
