(CNN) Recent satellite imagery shows vehicle activity at a critical North Korean missile site that could potentially signal early preparations for a missile or missile engine test, according to one administration official and three defense officials familiar with the latest US assessment.

Vehicles have been seen at the Sanumdong missile research center near Pyongyang in recent days, the officials said. The vehicles are not believed to be involved in fueling missiles and US officials are not certain whether North Korea may be moving toward the launch of a short-or-medium-range missile or an engine test.

The "activities are consistent with what we've seen prior to other missile tests," one senior US official said. The officials, however, said there is no indication of an imminent test launch but added as always they cannot rule it out.

Intelligence analysts are not ready to rule out a test could be imminent, but the assessment is that North Korea would have to undertake several steps, including visible fueling, before launching a long-range missile. Sanumdong has been associated with the regime's long-range missile program.

Vehicle activity at Sanumdong has been intermittent in recent months. Researchers at The Middlebury Institute, who have monitored the site since 2017, told CNN the recent activity appears inconclusive and cautioned that the North Koreans are well aware their sites are monitored by US satellites so it could be part of an effort to mislead US intelligence.

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