Satellite images suggest North Korea could be in the final stages of preparations for a new nuclear test according to US-based analysts, who echoed similar conclusions by the US military.
The images, taken Saturday, show up to four vehicles or equipment trailers continuously present at the entrance to the Punggye-ri Nuclear Test Site, with communications cables likely laid on the ground.
"This equipment would likely be used to initiate the test, collect data from the explosion and process the data," said 38 North, a project of the US-Korea Institute at Johns Hopkins University.
Last week, the US military reached similar conclusions after observing activity at the hermit state's nuclear sites.
Pyongyang is on a quest to develop a long-range missile capable of hitting the US mainland with a nuclear warhead, and has so far staged five nuclear tests, two of them last year.
Another blast would be a fresh challenge for new US President Donald Trump, who has tweeted that its goal of possessing an intercontinental ballistic missile "won't happen."
The North carried out multiple missile launches in 2016, and earlier this month sent up four rockets in what it said was a drill for an attack on US bases in Japan.
Soon afterwards new US Secretary of State Rex Tillerson visited the region and said that 20 years of efforts to denuclearise the North had "failed".
He promised a new approach, without offering specifics.
The 38 North researchers noted Tuesday that water was being pumped out of the portal at Punggye-ri and drained downhill to keep the tunnel dry for monitoring or communications equipment.
"The combination of these factors strongly suggests that test preparations are well under way, including the installation of instrumentation," the researchers said.
But they warned that there was "no definitive evidence", adding: "Since North Korea knows the world is watching and is capable of deception, caution should be used before declaring that a nuclear test is imminent."
Seoul's defence ministry would not be drawn on details of the latest report, but said it was "keeping a close watch over the North, using surveillance assets of both South Korea and the United States".
A spokesman re-iterated its view that "North Korea is prepared to conduct nuclear tests any time when its leader decides to do so."
Despite a string of United Nations sanctions since it first tested a nuclear device in 2006, Pyongyang has insisted it will continue its program.