"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.
North Korea has conducted three underground, nuclear test explosions since 2006. The latest, in February 2013, prompted international condemnation and a tightening of sanctions by the U.N. Security Council. The sanctions are intended to hinder the North's development of nuclear weapons and ballistic missiles.
The North's weapons capabilities are a subject of conjecture, but experts doubt it has yet mastered how to miniaturize a nuclear device that fit on a long-range missile capable of hitting the United States, although it is possible it may have for shorter-range missiles.