Satellite images taken on Wednesday include hints that North Korea may be preparing to detonate multiple nuclear devices at once.
The new surveillance photographs reveal "increased movement of vehicles and materials near what are believed to be the entrances to two completed test tunnels" in the southern part of the testing site, said Jack Liu in a Thursday image analysis for 38 North, an expert website that tracks weapon developments in North Korea.
Pyongyang has warned repeatedly in recent weeks that it is prepared to conduct a "new" kind of atomic test. The detected movement around two different tunnels at the Punggye-ri test site could mean North Korean personnel are placing atomic devices in each tunnel, though that is by no means certain.
Relying on commercial satellites to draw understanding of what events could be taking place underground can be challenging, and longtime North Korea watchers are careful to emphasize there is no way of definitively knowing how North Korea's next nuclear test might play out.
"One way of them demonstrating a new form of testing would be to conduct multiple tests at one time, just like the Pakistanis did in 1998," said Joel Wit, editor of 38 North, in a Friday phone interview.
He stressed, though, that he "can't say that these pictures show that that is what they are doing. I can't jump to that conclusion."
Jeffrey Lewis, director of the East Asia Nonproliferation Program at the James Martin Center for Nonproliferation Studies, in a March analysis for 38 North speculated that recently detected excavation work at Punggye-ri could have been to support an effort "to transform the mountains north and south of the site into complexes that could allow [for] multiple tests -- two or more -- in drifts off a single main tunnel."
Another way that North Korea could implement its threat of a new kind of nuclear test would be to detonate -- for the first time -- a device fueled by highly enriched uranium, instead of the plutonium it has used in the past, experts say.
Liu, in his Thursday image analysis, also detected the presence of likely command-and-control vehicles at Punggye-ri's Main Support Area that could have been brought over to supply secure communications between the test site and other parts of North Korea. He notes that "similar vehicles were spotted in the area" shortly before the country carried out its last nuclear explosion in February 2013.
One of the final indications that a nuclear test could be imminent would be when North Korea seals the entrance to a tunnel with cement or dirt. 38 North did not say it had detected such activity, but a South Korean government official told reporters on Thursday that Pyongyang is believed to have closed off a test tunnel.
U.S. President Obama and South Korean President Park Geun-hye on Friday warned the North that harsh consequences would follow if it carried out a fourth nuclear test, the Associated Press reported.
Another trial blast would cause a "fundamental change" in the region's security dynamics, prompting other nations to push to develop equal atomic capabilities, said Park during a joint press conference with Obama, who will be in Seoul until Saturday.
She also indicated that multinational negotiations aimed at achieving a permanent North Korean denuclearization would be permanently called off.
Obama warned that another nuclear test could lead to new sanctions with "even more bite."