Further evidence of a new portal at North Korea’s nuclear test site. (Courtesy of 38 North/Airbus Defense & Space/Spot Image)

North Korea appears to be building a new tunnel at its nuclear test site, new satellite pictures show, raising fears that the Pyongyang regime might be preparing to conduct a fourth atomic test.

Although there are no signs that a test is imminent, the construction does lead to one unpalatable conclusion, said Jeffrey Lewis, a respected nonproliferation expert at the Middlebury Institute of International Studies at Monterey.

“Well, they’re not giving up their nuclear weapons,” said Lewis, who analyzed commercial satellite imagery in a new report for 38 North, a website dedicated to North Korea run by the U.S.-Korea Institute at Johns Hopkins University’s School of Advanced International Studies.

The new tunnel makes it “more likely that they will conduct a test in the coming year,” Lewis said, although this also depended on factors such as stocks of fissile material and the political situation.

Since 2006, North Korea has conducted three nuclear detonations at its Punggye-ri test site, in a mountainous area in the north of the country. The most recent was in February 2013, just over a year into Kim Jong Un’s reign and a month before Xi Jinping became China’s president, putting a bitter chill on relations with North Korea’s neighbor and reluctant patron.

Portals at North Korea’s Punggye-ri nuclear test site. (Courtesy of 38 North/Airbus Defense & Space/Spot Image)

Kim has repeatedly asserted North Korea’s status as a nuclear-armed country and has resolutely refused to return to multilateral talks aimed at persuading it to abandon its nuclear weapons program. But, to the surprise of many analysts, there has been no fourth test.

Recent satellite photos show that North Korea is excavating a new tunnel at the test site.

Significant construction, including of new covered buildings, began in April, Lewis wrote. By October and November, the satellites were showing an additional structure and what appear to be significant tailings, indicating excavation of a new tunnel is underway.

South Korean officials have also reached the same conclusion. “North Korea appears to be in the process of digging another tunnel,” an unnamed official told the Yonhap News Agency in late October, citing increased movement of people and cars in the area.

But Joel Wit, a former American diplomat who is now a visiting scholar at the U.S.-Korea Institute, warned against concluding a test was coming anytime soon, noting the construction had been going on for months.

“There’s no evidence that they have decided to conduct a test,” Wit said, “but it adds to their ability to do so.”

There are already three tunnels at the Punggye-ri site: the east portal, where North Korea conducted its first nuclear test in 2006 and which does not appear to be maintained; the north portal that was used for the 2009 and 2013 tests; and the south portal, which has been under construction for six years and has not been used.

If the new tunnel, which Lewis called the west portal, connects to the same main tunnel, Pyongyang would be able to conduct additional tests in the future, he said.

Analysts must couch their assessments with “coulds” and “mays” because so little is known about North Korea’s nuclear program. Almost all information comes from satellites in orbit overhead, earthquake detectors that register the seismic activity caused by the detonations and spreadsheets.

The Washington-based Institute for Science and International Security estimates the North Korean regime could have enriched enough plutonium and uranium for 22 nuclear weapons.

In this case, Lewis said that a constraint on testing could be the mountain itself: whether it’s big enough to contain repeated explosions or whether it is suffering from “tired mountain syndrome” as the rock weakens.