The images show steam rising from a re-processing plant at the North's main Yongbyon nuclear complex -- a sign consistent with maintenance and testing prior to commencing operations, the US-Korea Institute at Johns Hopkins University said on its closely followed 38 North website.
The facility is used to reprocess spent fuel from the five-megawatt reactor at Yongbyon that is North Korea's main source of weapons grade plutonium.
While warning it was still early to reach a definitive conclusion, the institute said evidence suggested the shutdown may have allowed the removal of "a limited number" of fuel rods for possible re-processing.
The images also showed truck activity near the vehicle door to the building that receives the spent fuel at the reprocessing complex, it said.
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The new analysis coincided with fresh threats from Pyongyang to carry out a new underground nuclear test following the UN adoption of a landmark resolution that condemns North Korean rights abuses.
The resolution was slammed as "counter-productive" today by Russia, which also shrugged off reports that Pyongyang may be firing up a facility to process weapons-grade plutonium.
"First of all, it is best to put forward such statements when they are based on facts -- not through the media," Russian Foreign Minister Sergei Lavrov told reporters after talks with a special envoy of North Korean leader Kim Jong-Un.