“Significant tunneling” has been detected at North Korea’s Punggye-ri nuclear test site, according to a Washington-based think tank specializing in tracking the Hermit Kingdom's activities.

Satellite images from December show increasing efforts to maintain the test site where North Korea has conducted the last six underground nuclear tests, claims a report Thursday by 38 North. The publication is run by Johns Hopkins University.

News of a tunnel reportedly crumbling at the nuclear site in October – likely because of the country's sixth and most powerful nuclear test in September, which potentially killed 200 people -- sparked a lot of speculation about the site's future.

But the analysis of satellite images suggests that although the North Portal of the site – where the last five nuclear tests were conducted – remains “dormant,” tunnel excavation in the West Portal has been “stepped up.”

“Throughout December 2017, mining carts and personnel were consistently present around the West Portal and there was significant expansion of the spoil pile,” the report says. There also appear to be new rails on top of the spoil pile.

“On December 28, there were also a large number of personnel (~100 to 120) observed in seven different formations whose purpose is unknown in the Southern Support Area,” it adds.

“It is rare to observe personnel in this area,” the report says.

The report concludes that such activities “underscore North Korea’s continued efforts to maintain the Punggye-ri site’s potential for future nuclear testing.”

News of apparent active nuclear test site comes just days after North Korean officials met with South Korean officials for the first time in more than two years.

On Tuesday, North Korea agreed to send a delegation to next month’s Winter Olympics in the South and to discuss easing military hostilities.

The communist regime in the North has long been boasting about the continuing development of nuclear weapons and its strike capabilities. In November, the regime said its new intercontinental ballistic missile can carry a “super heavy nuclear warhead” that could reach the U.S. mainland.

The Associated Press contributed to this report.