North Korea has been restoring access to a yet unused tunnel at the Punggye-ri nuclear test site in a potential attempt to expedite a seventh nuclear weapons test, said Open Nuclear Network, a Vienna-based research institute.

Commercial satellite images have shown “signs of increasing activities” since last December especially at the South Portal and the main administrative area within North Korea’s only known underground nuclear test site, according to the institute’s research report released on Monday.

“These developments clearly show that the DPRK has reactivated, or is in the process of reactivating, some parts of the Punggye-ri nuclear weapon test site, especially the South Portal and the main administrative area,” Dr. Katsuhisa Furukawa, a senior analyst for Open Nuclear Network and former member of the UN Panel of Experts on North Korea, said in the report.

The Punggye-ri nuclear test site had “remained almost inactive” since in May 2018 when North Korea had dismantled all of the four portals or tunnels, except for the East Portal shuttered after the first nuclear test.

But since March, the satellite imagery has shown more ”visible“ indications of Pyongyang restoring access to the South Portal, also known as Tunnel 3, at the nuclear test site in Punggye-ri in Kilju County of North Hamgyong Province.

A total of four tunnels, which are referred to as the East, North, South, West Portals, were constructed at different times between 2006 and May 2018 to conduct nuclear tests.

Pyongyang used the East Portal for its first nuclear test in October 2006 and North Portal for the rest of the five nuclear weapons tests carried out from May 2009 and September 2017. The country has not yet conducted any nuclear tests at the West and South Portals up to date.

Open Nuclear Network pointed out that the “signs of excavation were visible close to the secondary entrance to the South Portal in March,” while there have been no indications of excavation and activities at the other three tunnels.

The satellite images from March 23 and 24 showed “several signs of excavation activities were observed near the secondary entrance.” For example, an installation of what could be a road became “visible” in that area, and seemed to connect to the likely secondary entrance or a possible new entrance to the South Portal. Indications of movements of vehicles or personnel were also visible in the imagery.

“The DPRK has very likely already started its efforts to restore the secondary entrance to the South Portal or establish a new entrance nearby,” the report said.

The satellite imagery from this month also showed indications of construction and renovation activities, “increased vehicle traffic,” and accumulated soil piles, especially near the secondary entrance to the South Portal.

North Korea, for instance, seemed to have partially repaired the roof of the semi-destroyed building located between the main administrative building and the South Portal.

A new building and a new visible object were seen constructed in the same place of the two now-shuttered buildings that had been linked to the excavation of the primary and secondary entrances to the South Portal.

Within the main administrative area, the satellite imagery last week showed possible but “visible” log piles that could be “used for the construction of building (s) or restoration of the demolished tunnels.”

The “increased vehicle traffic” was observed in the main administrative area and between the area and the South Portal.

The research institute agreed with the previous assessment by South Korean authorities that North Korea could restore Tunnel 3 “within a month” with the goal to expeditiously prepare for the seventh nuclear test.

“Analysis of available satellite imagery is consistent with this hypothesis, indicating that the DPRK’s excavation efforts have likely started in order to restore the South Portal for use in an upcoming nuclear weapon test,” the report read.

“If the tunnel structure inside this portal was not damaged extensively in May 2018, it might be possible for the DPRK to rapidly restore access to the tunnel through the secondary entrance.”

The detected activities could be a sign of forthcoming nuclear tests, especially at a juncture when North Korea last week renounced its self-imposed moratorium on testing an intercontinental ballistic missile.

“It is assessed as likely that the DPRK may also end its moratorium on nuclear weapons testing in the near future.”

At the eighth party congress, North Korean leader Kim Jong-un in January last year called for advancing nuclear technology, miniaturizing nuclear warheads, developing tactical nuclear weapons and pushing ahead with the “production of supersized nuclear warheads.”