North Korea appears to be at work to reactivate another unused tunnel at the Punggye-ri nuclear testing site, and has completed preparations for an oft-speculated nuclear test in Tunnel No. 3, a Washington-based think tank said Wednesday.

Commercial satellite images acquired Tuesday show new indications of construction activity below the entrance to Tunnel No. 4 at the nuclear test site in Punggye-ri in Kilju County of North Hamgyong Province.

A new caisson wall is under construction and construction materials have been observed near the entrance to Tunnel No. 4, which is also known as the West Portal, according to the analysis of the latest satellite imagery.

In the run-up to the first US-North Korea summit, North Korea in May 2018 had dismantled the four portals or tunnels except Tunnel No. 1, the East Portal, which has been closed since the first nuclear test in October 2006.

“New construction activity is noted at Tunnel No. 4, strongly suggesting an effort to reenable it for potential future testing,” according to the CSIS Korea Chair, which runs the Beyond Parallel project and compared satellite images collected on May 17.

“The extent of actual damage inside the tunnels due to the disabling was unclear, and these new indicators of activity suggest that the disabling was not complete, as is the case with Tunnel No. 3,” the report read.

The CSIS Korea Chair further explained that the ongoing activities in the main administration and support area observed in the satellite images could be “indicators of the further reenabling of the facility at Tunnel No. 4.”

In the area, the refurbishment of former support and storage buildings and the construction of several new buildings have continued. Vehicle tracks are also visible throughout the courtyard.

But the South Korean military reportedly views the construction activity near Tunnel No. 4 as related to repairing unused roads that were washed away in heavy rain, The Korea Herald learned. The military also says it is too early to conclude that the new construction activity will lead to the restoration of access to Tunnel No. 4.

Additionally, the refurbishment work and preparations at Tunnel No. 3, the South Portal, are “apparently now complete and ready for an oft-speculated seventh nuclear test,” according to the CSIS Korea Chair.

“The timing of a seventh nuclear test now rests solely within the hands of Kim Jong-un,” it said, explaining that work at Tunnel No. 3 began around four months ago.

No significant activity has been observed in the vicinity of Tunnel No. 3 in the satellite images. But the actual concrete portal with an adjacent caisson retaining wall and some minor landscaping with small trees or bushes seems to be part of preparations for a visit by senior North Korean officials.

Seoul and Washington have said North Korea is readying for a seventh nuclear test. South Korea’s Foreign Minister Park Jin on Monday said North Korea “has now finished the preparation for another nuclear test” during a joint press conference with US Secretary of State Antony Blinken.

The head of the International Atomic Energy Agency earlier this month also said it has observed indications that one of the tunnels has been reopened, “possibly in preparation for a nuclear test.”

A total of four tunnels were constructed at different points between 2006 and May 2018 at the Punggye-ri nuclear testing site.

Pyongyang used the East Portal for its first nuclear test in October 2006 and the North Portal for its other five nuclear weapons tests carried out between May 2009 and September 2017. But it has not yet conducted any nuclear tests using the West and South Portals to date.