Satellite imagery shows steam rising from North Korea's Yongbyon nuclear facility, suggesting the reactor has been restarted.
In April, Pyongyang announced its intention to restart the facility. The report, written by the US-Korea Institute at Johns Hopkins University, used satellite images dated August 31.
"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," report author Jeffrey Lewis told the BBC.
The five-megawatt reactor uses steam turbines to generate electricity, and can produce plutonium, which can be used to make nuclear weapons.
Analysts believe North Korea already has between four and 10 nuclear weapons, produced at the facility before it was shut down in 2007 as part of a deal to disarm in return for aid.
"They really are putting themselves in a position to increase the amount of material they have for nuclear weapons, which I think gives them a little bit of leverage in negotiations, and adds a sense of urgency on our part," Lewis said.
The reactor produces spent fuel rods that can be made into plutonium, and analysts believe the Yongbyon reactor can produce about 13 pounds of plutonium per year, enough for one or two nuclear bombs.
But the Institution for Science and International Security says it would take time before any new plutonium would be usable.
"Given that North Korea will likely need two-three years before it discharges irradiated fuel containing plutonium and another six to 12 months to separate the plutonium, there remains time to negotiate a shutdown of the reactor before North Korea can use any of this new plutonium in nuclear weapons," the think tank said in a report.