TOKYO — North Korea could be taking preliminary steps to close its nuclear test site, according to new satellite images that suggest Kim Jong Un might be making good on one of the surprising pledges he’s made over the past month.

Or, he’s making the rest of the world think he is by arranging a performance for the satellites that pass overhead.

Satellite images taken since last month’s inter-Korean summit show a steady reduction in the number of buildings around North Korea’s known nuclear test site, built under Mount Mantap in the Punggye-ri area in the north of the country.

“At the very least, this is a welcome PR move,” said Jeffrey Lewis, head of the East Asia program at the James Martin Center for Nonproliferation Studies in Monterey, Calif.

“Over the past two weeks, five or six buildings have inexplicably come down,” Lewis said, citing commercial satellite images from the San Francisco-based firm Planet Labs that have a resolution comparable to Google Maps. “Something is clearly happening there.”

As part of the extraordinary rapprochement going on, North Korea has vowed to dismantle the test site, where all six of its nuclear detonations have taken place, this month. But as with so many things about North Korea, it’s difficult to tell how much of this is wheat and how much is chaff.

Kim made the pledge during a historic summit with South Korean President Moon Jae-in, which laid the groundwork for a meeting between Kim and President Trump, which Trump announced Thursday would take place June 12 in Singapore.

President Trump announced via Twitter on May 10 that he will meet with North Korean leader Kim Jong Un on June 12 in Singapore. (Reuters)

 Kim said he would invite security experts and journalists to the North to observe the closure of the site, the South’s presidential Blue House said.

All six of North Korea’s nuclear tests have taken place deep under the mountain at Punggye-ri, with five of them occurring in the tunnel complex accessed through an entrance known as the north portal. There are two other entrances to the site, the west and south portals.

The last test, in September last year, was so huge that some experts wondered whether Mount Mantap was suffering from “tired mountain syndrome” and had become unusable. But nuclear experts have cast doubt on that theory, noting that even if the tunnels leading to the north portal were unusable, the other two entrances could still be operational.

Tunneling and activity at the west portal had been visible as recently as April 20, a week before the inter-Korean summit, according to an analysis for 38 North, a website devoted to North Korea.

There are clusters of buildings at the portals, including administration buildings and a command center, as well as smaller buildings.

The big, main buildings are still there but the smaller, more peripheral ones at the north and south portals, the entrances to the main tunnels, have come down, Lewis said. 

This could be part of the preparations for inviting journalists and experts to watch the closure of the site, which, Lewis said, could be as simple – and as reversible – as blocking the portals. 

“Shutting down the test site is something they can easily do. It’s just tunnels so they can seal the entrances – but they can also unseal them,” he said.

“And the tunnels are always going to be there,” he added, unless North Korea blows up the whole site.

Still, analysts wanting to be optimistic about the diplomatic process say that declaring the site finished and taking steps toward closing it would support their theory that Kim is making an effort, just like this week’s release of three Americans who had been held in North Korea.

But skeptics say that closing a test site that might well be spent is just cosmetic.

A group of Chinese scientists last month said they believe that the test site collapsed after September’s huge test, which caused an earthquake so big that satellites caught images of the mountain above the site moving.

North Korea claimed to have detonated a hydrogen bomb, which would be exponentially more powerful than the atomic devices previously tested, and experts said the size of the earthquake suggested that it had indeed been a hydrogen, or thermonuclear, explosion.

Adding to the theory that the site has outlived its utility is new research from scientists from the Earth Observatory of Singapore at Nanyang Technological University, who claim to have found evidence that the damage at Mount Mantap was more substantial than other research shows.

In their study, which will be published in the journal Science on Thursday, they argued that by using satellite radar imagery to supplement ground-based seismological readings, they were able to gain a more accurate picture of the Sept. 3 test.

Sylvain Barbot, a researcher with the Earth Observatory of Singapore, wrote in an email that the nuclear test last year was so large that “we could ‘feel’ it from space.” 

The amount of shaking that accompanied the explosion was so severe that traditional radar measurements were inaccurate, Barbot said, and his team had to use unusual techniques to compensate for significant changes in the landscape.

By using these techniques, the researchers were able to estimate a depth for the nuclear detonation: around 450 meters, or roughly 1,500 feet, beneath the summit of Mount Mantap. Researchers then combined this information with seismological readings to come up with an estimated yield for the weapon of 120 to 304 kilotons.

Much of this range would be far higher than officials from the United States and South Korea estimate.

The researchers also found evidence that a significant part of Mount Mantap had collapsed after the explosion, supporting the Chinese study. A “very large” part of the facility had collapsed, Barbot said, “not merely a tunnel or two.”

Taylor reported from Washington.