New satellite imagery taken Tuesday shows a gathering of up to 100 people at the administrative area of North Korea’s main nuclear test site — a level of activity not been seen since January 2013, a month before Pyongyang’s third atomic test, the influential 38 North website said Wednesday.

The large gathering of people apparently standing in formation or watching an event at the Punggye-ri nuclear test site is the latest development in a series of recent signs that the North may be readying to conduct its sixth nuclear test, said 38 North, a project of the U.S.-Korea Institute at Johns Hopkins University.

In a report a day earlier analyzing satellite images taken Saturday, the website said there were strong suggestions that preparations for a nuclear test are entering their final stages. These included the presence of vehicles and the apparent laying of communications cables at the entrance to an underground test tunnel. The report said water was also being pumped out of the tunnel or being drained downhill, presumably to keep it dry for monitoring or communications equipment.

“The combination of these factors strongly suggests that test preparations are well under way, including the installation of instrumentation,” 38 North said in the earlier report.

Wednesday’s report said that this activity was continuing, noting that the North could be trying to send a political message. The report, however, offered the caveat that the North could be seeking to deceive observers.

“The North Koreans know when commercial satellites are passing overhead and typically try to avoid activities during that time,” it said. “The fact these formations can be seen suggests that Pyongyang is sending a political message that the sixth nuclear test will be conducted soon. Alternatively, it may be engaged in a well-planned game of brinkmanship.”

The website had also noted last week that the presence of vehicles at the test facility could indicate that instruments — and even a nuclear device — may have been installed for an underground blast.

A sixth nuclear test by the North would come amid a flurry of activity by the country as it seeks to master the technology needed to mount a nuclear weapon on a long-range ballistic missile capable of striking the continental United States.

Some experts have said that more than a fresh nuclear test, the more pressing concern is the North’s increasingly capable missile program.

“They have a nuke. It works. It’s bigger than the U.S. nukes used in World War II. That’s established now,” said Robert E. Kelly, a professor of international relations at Pusan National University in South Korea. “It’s bad, to be sure, that they keep testing. And if it is the rumored step-up to a fusion weapon, that is a big deal. But I am now more interested and worried about things like miniturization, guidance, range.”

Kelly said a fusion device or hydrogen bomb would have a yield “an order of magnitude greater.”

“That would be pretty damn big,” he said. “But ultimately the issue is not so much size now — any nuke in a city will be hugely destructive — it’s delivery. Can they get it there? So I’m watching for more multistage rocket tests.”

There has been growing speculation that Pyongyang will conduct an intercontinental ballistic missile test after leader Kim Jong Un used a New Year’s Day address to claim that the North was in the final stages of developing such a weapon.

Any nuclear or ICBM test would pose a fresh challenge to U.S. President Donald Trump, who has vowed that Pyongyang’s goal of possessing a nuclear-tipped long-range missile “won’t happen.”

U.S. Secretary of State Rex Tillerson said this month during a visit to Asia that years of efforts to get the North to give up its nuclear weapons program have failed. Tillerson promised “a new approach,” saying that military action against the North “was on the table.”

The international community has been piling pressure on the North after it conducted two nuclear tests and launched more than 20 missiles last year.

This month, Pyongyang also issued an unusually overt threat to Washington and Tokyo, firing off a simultaneous barrage of four missiles as part of exercises that it said were training for strikes on U.S. military bases in Japan.

Prime Minister Shinzo Abe characterized the exercise as exemplifying “a new level of threat” from the North.

Missile experts said the hypothetical target of that drill appeared to be U.S. Marine Corps Air Station Iwakuni in Yamaguchi Prefecture. Observers said the undisguised threat to U.S. bases in Japan was rare, even for Pyongyang, which routinely serves up colorful invectives.

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