South Korea's National Intelligence Service also said it could not confirm the reactor report because it involves confidential intelligence.
Commercial satellite images taken in recent months had indicated preparations for restarting the reactor were progressing rapidly. In June, the institute predicted the reactor could be up and running by the end of August, depending on the availability of fresh fuel rods to power it.
The reactor can be used both to generate electricity and produce fissile material for nuclear weapons, and once it is operating, can produce about 6 kilograms of plutonium a year — enough for one or two bombs. Experts estimate North Korea already has enough plutonium for between four to eight crude weapons.
"The reactor restart fits a pattern of continued expansion of North Korea's WMD programs short of conducting outright nuclear and missile tests," said Joel Wit, a former State Department official and editor of 38 North. WMD stands for weapons of mass destruction.
"An operating reactor will enable Pyongyang to renew the production of plutonium, albeit on a small scale, that will enable it to slowly expand its stockpile of nuclear bombs," he said.
The development could intensify pressure on Washington to restart international aid-for-disarmament negotiations with Pyongyang, stalled since 2009, although the differences between them are stark.
The U.S. is demanding North Korea first recommit to past agreements on denuclearization, while Pyongyang insists that it be recognized in disarmament negotiations as a nuclear power, which the U.S. refuses to do.
North Korea has dialed down its bellicose rhetoric in recent months and on Wednesday agreed with South Korea to restart operations at a jointly run factory park that Pyongyang shut down in April.
But despite the easing tensions, analysis of recent commercial satellite photos also shows Pyongyang may also be doubling the size of its uranium enrichment plant — a potential, second source of fissile material for nuclear weapons — and expanding its main rocket launch site, located on its west coast near China.