Last month, the Bulletin of the Atomic Scientists stated the completion of a tunnel near the site of North Korea's first nuclear tests in 2006 and 2009 suggested Pyongyang might be preparing for another test.
Analysts at IHS Jane's Defense Week told The New York Times that North Korea made good progress on the construction of a light-water uranium reactor.
Enriched uranium could find its way into the North Korea nuclear arsenal. The country's first nuclear tests were believed to have used plutonium devices.
International Atomic Energy Agency Director General Yukiya Amano said the IAEA was concerned about the pace of nuclear development in North Korea, Russian news agency ITAR-Tass reports.
Without access to the country's light-water reactor, Amano said it was difficult to make assessments about the condition of the facility.
Missile tests by North Korea coincided with the country's first two nuclear tests. Nuclear concerns were raised following an April effort by Pyongyang to use a long-range rocket to send a satellite into orbit.