The US director of national intelligence released a long-awaited report on Friday into incidents involving so-called UFOs, concluding that more data collection is needed to identify the phenomena.
The report said 143 of 144 incidents of aircraft or flying objects that were not immediately identifiable between 2004 and 2021. The government called the incidents "unidentified aerial phenomena".
Most of the incidents were reported in the last two years as reporting improved, the report said.
Just one of the incidents was identified with high confidence: it was a large, deflating balloon, the intelligence report said.
But there were 18 incidents that involved unusual "movement patters or flight characteristics".
These included objects that "appeared to remain stationary in winds aloft, move against the wind, manoeuvreabruptly, or move at considerable speed, without discernable means of propulsion," the report said, adding that they also demonstrated "advanced technology."
The incidents could fall into five different categories: airborne clutter, natural atmospheric phenomena, classified US programmes, foreign adversary systems from countries such as China and Russia, or other.
The report which was sent to US Congressional committees said the "limited amount of high-quality reporting" on the phenomena "hampers our ability to draw firm conclusions."
UFOs have long been part of science fiction folklore but recently officials have highlighted the national security concerns surrounding the unidentified phenomena, particularly near defence sites.
"It is critical that the United States maintain operations security and safety at DoD ranges," US deputy secretary of defence Kathleen Hicks said in a memo released after the report.
"To this end, it is equally critical that all U.S. military aircrews or government personnel report whenever aircraft or other devices interfere with military training."
She said the mission to identify the phenomena would be formalised within the Department of Defence.