The Bulletin of the Atomic Scientists last week said satellite imagery suggested a tunnel completed near the site of the country's nuclear tests in 2006 and 2009 indicated North Korea might be capable of another nuclear test soon.
A recent survey by IHS Jane's Defense Weekly, meanwhile, said North Korea was progressing with the construction of a light-water uranium reactor.
Victoria Nuland, a spokeswoman for the U.S. State Department, said Washington maintained that North Korea was obliged to address international concerns.
"The concerns with regard to their nuclear program are not changed," she said. "They remain as they were."
Nuclear tests in 2006 and 2009 used plutonium devices. Enriched uranium is also used in nuclear weapons.
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.