Satellite images of the Yongbyon plant, the regime’s primary nuclear research centre, show extensive plumes of vapour coming from the generator hall.
Experts from the site 38 North, which published the images, say it points to Kim resuming his nuclear programme.
They wrote: “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.
“It also means that the North has likely extended its cooling water pipeline into the river to better conceal the reactor’s operational status, making monitoring efforts more difficult going forward.”
A new military camp has also been observed near the site which could be for a new construction project.
Two flatbed trucks with blue tarpaulins have also been spied, prompting speculation they are being used to deliver nuclear material.
The news comes after Pyongyang warned the US it was aggravating the situation on the Korean peninsula and bringing “dark clouds of a war”.
The images were taken in late February, shortly before this week’s meeting between Kim and South Korean government officials.
Kim sat down with the delegation for the first time yesterday and said it is his “firm will to vigorously advance” inter-Korean ties and pursue reunification.
A 10-member South Korean group led by National Security Office head Chung Eui-yong travelled to the secretive state in hopes of encouraging North Korea and the US to talk to one another.
Today it was announced the uneasy neighbours will hold their first summit in more than a decade in late April.
In a bid to appease the West, Pyongyang said there was no need to keep its nuclear programme if the safety of the hermit state was secured.
North Korea also said it was open to talking with the United States over denuclearisation and normalising ties.