Steam has been seen rising from North Korea's Yongbyon nuclear facility, suggesting that the reactor has been restarted.
The US-Korea institute at Johns Hopkins University said on Wednesday the colour and volume of the steam indicated that the reactor was in or nearing operation.
Pyongyang vowed to restart facilities at its main Yongbyon nuclear complex in April, amid regional tensions.
The BBC reports the reactor can produce plutonium, which North Korea could use to make nuclear weapons.
Analysts believe North Korea already possesses between four and 10 nuclear weapons, based on plutonium produced at the Yongbyon reactor prior to mid-2007, when the facility was closed.
The report said the reactor uses steam turbines to generate electricity and steam seen in satellite imagery from 31 August indicated that the electrical system was about to come online.
"The reactor looks like it either is or will within a matter of days be fully operational, and as soon as that happens, it will start producing plutonium," author Jeffrey Lewis told the BBC.