The website 38 North, which monitors commercial satellite imagery for clues to North Korea's actions, says the continued activity in many areas of the Punggye-ri site shown by images from February 18 indicate that work is underway at many of the site's tunnels.
"In particular, the shifting of supplies and equipment, as well as additional changes in the texture and pattern of small sections of the tailings pile at the North Portal, indicate continuing work inside the test tunnels," 38 North said.
"These activities suggest continued preparation and maintenance of this portal for use in a new nuclear test. Should the decision be made to do so, North Korea could probably move forward with a nuclear test in short order but it remains unclear when such a test might take place."
With the snow having been cleared in some areas around the site, a "sizable collection of supplies or equipment in the northern courtyard," has been revealed, the website notes.
Tensions are high between the reclusive nuclear state and most of the rest of the world at the moment. Following North Korea's last intermediate-range missile test, China announced it would slash coal imports from the country, one of its last sources of income. The murder of Kim Jong-nam, half-brother of North Korean leader Kim Jong-un, in a Malaysian airport has caused many to conclude the Pyongyang government must be involved, and some US officials are calling for the country to be re-listed as a state sponsor of terrorism.
North Korea's bluster continues, however. The country lashed out at Japan for its "moves for overseas invasion" in an editorial in the Korean Central News Agency (KCNA).
"Japan is behaving ridiculously, finding fault with the DPRK's test-fire of Pukguksong-2 and talking about a drill of intercepting it and introduction of a new type hardware," KCNA wrote.
Japan and the US are to hold joint military drills next month. North Korea frequently gets testy when Japan practices intercepting its missiles, which North Korea likes to lob into the Sea of Japan.
"The moves stepped up by Japan in the wake of the DPRK's test-fire of Pukguksong-2 are being ridiculed by the world community as foolish attempts to cover its ambition for overseas invasion under the pretext of the former's missile test-fire," KCNA wrote, while complaining about Japan's supposed nuclear capabilities.
"It is the calculation of Japan to attain the position of the leading power in Asia even by offering its whole archipelago as an advance base of the U.S. for invading the continent while zealously following the U.S. hostile policy towards the DPRK. The Japanese reactionaries, however, should not miscalculate," Pyongyang warned.