A U.S. policy institute said it may have located a secret facility used by North Korea in the early stages of building its program to enrich uranium for nuclear weapons, which if confirmed would be critical to the success of any future nuclear deal, according to a report seen by Reuters on Thursday.

The report by the Institute for Science and International Security said there has always been doubt about whether North Korea has disclosed all of its nuclear facilities. Confirming their location would be critical to the success of any future agreement to freeze and dismantle North Korea's nuclear weapons program, it said.

The site, 27 miles from the nuclear complex at Yongbyon, may have played a key role in development of centrifuges that refine uranium hexafluoride gas into low-enriched and highly enriched uranium, the report said.

"It is necessary to identify where North Korea enriches uranium and part of that is understanding where it has done it in the past," said David Albright, the institute's president.

What may once have been the early centrifuge research and development facility is believed to have been inside an aircraft part factory inside a mountain next to Panghyon Air Base. It was located using commercial satellite imagery, the report said.

It was unclear whether the aircraft part factory was still operational but information from defectors indicates there may be three production-scale centrifuge manufacturing plants operating in the country although their locations have not been confirmed, said Albright.

Tensions have been escalating between North Korea and South Korea, the United States and Japan over Pyongyang's fourth underground nuclear test in January and a series of missile launches.

North Korea's nuclear program is based on highly enriched uranium and plutonium separated from spent reactor fuel rods.

The reclusive government, which for more than a decade denied having a gas centrifuge program, in November 2010 revealed the existence of a production-scale gas centrifuge plant at Yongbyon but insisted it had no other such facilities.

In June 2000 a Japanese newspaper quoted Chinese sources as saying a facility was located inside Mount Chonma, the report said. Information recently obtained from "knowledgeable government officials" suggested the undeclared facility was associated with an underground aircraft parts factory, it said.

Working with Allsource Analysis, which interprets satellite imagery, the institute determined it most likely was Panghyon Aircraft Plant, which made parts for Soviet-supplied fighters.

The report quoted an unidentified official as saying the site could have held between 200 and 300 centrifuges.