North Korea’s Sanumdong missile research center has seen a flurry of vehicle activity in recent days, according to U.S. officials, citing satellite imagery.

The vehicles are not believed to be fueling missiles, according to one administration official and three defense officials familiar with U.S. assessments, but the "activities are consistent with what we've seen prior to other missile tests," a senior U.S. official told CNN.

While analysts could not rule out that tests were imminent, Pyongyang would have to go through several more steps, including visible fueling, before the government is capable of launching long-range missiles, according to the officials, CNN said.

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Researchers with the Middlebury Institute also noted that North Korean officials are aware the site is monitored and that any visible activity at the site could be part of a deliberate effort to mislead surveillance.

"The unusual traffic is difficult to interpret. If it is a leadership visit to the factory, that could come either at the beginning or the end of the construction of an ICBM or space launcher," Jeffrey Lewis, director of the Institute's East Asia Nonproliferation Project, told CNN.

"The important thing is that there is an uptick in activity at the site, just as there has been at Sohae and other facilities," he said.

North Korean leader Kim Jong Un Kim Jong UnSatellite images indicate North Korea preparing for massive military parade South Korea warns of underwater missile test launch by North Korea Trump says he didn't share classified information following Woodward book MORE recently announced Pyongyang’s intention to develop “strategic” weapons to push back against “hostile” U.S. policy.