The new images showing emissions rising from a reactor, said to be from February 25, bring into question North Korea’s seriousness about reducing its nuclear capability.
Rob Munks, Editor of Jane’s Intelligence Review, said the light-water reactor “could be used for civilian electricity generation, its stated purpose, or diverted towards the nuclear program”.
He added that the nuclear reactor could be operational by the end of the year “with little warning”.
The allegations from Jane’s come just weeks after it was suggested that Kim Jong-un and Donald Trump could have face-to-face talks about nuclear disarmament of Pyongyang.
In 2017 the hermit state held a number of missile tests to check its nuclear capability.
By the end of the year the rogue state said it held weapons capable of reaching the whole of the US, causing alarm and panic across the globe.
The tests were coupled by an escalating war of words between Kim Jong-un and Donald Trump.
However, in January the Asian nation suspended its nuclear programme and appeared to reach out to the neighbouring South.
Relations between the two Koreas have since been on the rise with the pair competing together at the Winter Olympic games and holding a series of bilateral talks.
Since then the US have said they are planning to hold meetings with the Kim regime to improve relations.
Trump said: “The deal with North Korea is very much in the making and will be, if completed, a very good one for the world.
“Time and place to be determined.”
He also tweeted: “North Korea has not conducted a missile test since November 28, 2017, and has promised not to do so through our meetings.
“I believe they will honour that commitment!”
However, the evidence suggesting Pyongyang has continued to grow its nuclear capabilities could put the peace talks at risk.
According to 38 North, a part of the US-Korea Institute at Johns Hopkins University, the images from February showed “steam vapour plumes emanating from the generator hall and river ice melt”.
They added: “If the reactor is operating again, as the evidence suggests, it means North Korea has resumed production of plutonium presumably for its nuclear weapons program.”