Story highlights U.S. spy satellites are keeping a sharp eye on the Sohae Satellite Launching Station

North Korea says it's a place where the country launches satellites

U.S. and Japanese officials say they're concerned that rocket used to launch peaceful satellites could be used as an ICBM

(CNN) It's a site so secretive that an underground railroad line delivers parts into a movable building, hiding activity from view.

North Korea describes it as a place where the country launches satellites into space.

But U.S. spy satellites are keeping a sharp eye on North Korea's Sohae Satellite Launching Station at Tongchang-ri. And increased movement of equipment and personnel into the facility is raising renewed concerns in the U.S. military and intelligence community that something imminent could be in the offing.

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Overhead satellite imagery in recent days has shown the movement of personnel, rocket-related equipment and fuel into the facility in the north of the country, according to a U.S. official.

North Korea will likely say it's launching a satellite from the site, but U.S. officials say the rocket involved could be used as an intercontinental ballistic missile.