A long-awaited US intelligence report on dozens of UFO sightings said most could not be explained, but did not rule out that some could be alien spacecraft.
The unclassified report said researchers could explain only one of 144 UFO sightings by US government personnel and sources between 2004 and 2021.
These sightings often were made during military training activities.
Eighteen of those, some observed from multiple angles, appeared to display unusual movements or flight characteristics that surprised those who saw them, said the report.
These included, holding stationary in high winds at high altitude and moving with extreme speed with no discernible means of propulsion.
Some of the 144 might be explained by natural or human-made objects – such as birds or drones – cluttering a pilot's radar, or natural atmospheric phenomena, the report said.
Others could be secret US defence tests, or unknown advanced technologies created by Russia or China, it said.
Yet others appeared to require more advanced technologies to determine what they are, it said.
The sightings of what the report from the Office of the Director of National Intelligence calls unidentified aerial phenomena "probably lack a single explanation".
"We currently lack sufficient information in our data set to attribute incidents to specific explanations."
The report made no mention of the possibility of that some of the objects sighted could represent extraterrestrial life – but nor did it rule it out.
The military and intelligence communities have conducted research on the craft as a potential threat.
"UAP clearly pose a safety of flight issue and may pose a challenge to US national security," the report said.
Some could be US rival intelligence collection operations or represent other technology so advanced that the US military has nothing similar.
The report was ordered after more UFO sightings by military pilots became public, and pilot and radar videos leaked out showing unexplained flying objects behaving strangely.
It stressed that pilots and their aircraft are ill-equipped to identify out-of-the-ordinary objects in the skies.
The only one of the 144 incidents in the years covered by the report that was explained turned out to be a large deflating balloon.
The nine-page report released Friday did not discuss any specific incidents.
It was the public version of a more detailed classified version being supplied to the armed services and intelligence committees of Congress.
Mark Warner, chairman of the Senate intelligence committee, said the frequency of UFO reports "appears to be increasing" since 2018.
"Today's rather inconclusive report only marks the beginning of efforts to understand and illuminate what is causing these risks to aviation in many areas around the country and the world," Mr Warner said.
"The United States must be able to understand and mitigate threats to our pilots, whether they're from drones or weather balloons or adversary intelligence capabilities."
