North Korea may be just one or two months away from restarting a reactor to produce plutonium, potentially allowing the regime to ramp up its nuclear weapons program, a think tank said Monday.

Analyzing satellite images, the US-Korea Institute at Johns Hopkins University said that North Korea has made "significant progress" at its Yongbyon site including on a five-megawatt gas-graphite reactor.

"The five-megawatt reactor may be one to two months from start-up but the availability of fresh fuel rods to power the reactor remains uncertain," said the institute's blog, 38 North.

"Once operational, the facility will be able to produce approximately six kilograms of plutonium per year that can be used for manufacturing nuclear weapons," it said, while cautioning that the lack of fuel rods could impede efforts.

The reported progress on the reactor comes after a slight easing of tensions with the totalitarian state, which in February carried out its third nuclear test and threatened war against the United States.

North Korea, led by young leader Kim Jong-Un, has repeatedly threatened to bolster its nuclear capabilities in response to what it perceives as hostility from the United States, which stations troops in democratic South Korea.

Recent satellite images have indicated that North Korea likely has completed a cooling system that is vital to run the reactor. North Korea knocked down a cooling tower in 2007 to show its commitment to a US-backed denuclearization plan.

The latest images, analyzed by experts Jeffrey Lewis and Nick Hansen, found that North Korea may also be just months away from starting up a light-water reactor at Yongbyon, with work apparently underway to finish the interior.

Light-water reactors are generally for civilian energy, but the United States, South Korea and Japan have voiced concern about North Korea's intentions.