A major US government report on UFOs found 143 of 144 "sightings" were unable to be explained.
The Department of Defense set up the Unidentified Aerial Phenomena (UAP) Task Force last August to investigate mysterious flying objects observed by military pilots.
Submitted to Congress and released to the public, the report includes some cases that previously came to light in the Pentagon's release of video from US naval aviators, showing an enigmatic aircraft off the US East and West Coasts.
The aircraft was said to be exhibiting speeds and maneuverability exceeding known aviation technologies and lacking any visible means of propulsion or flight-control.
A senior US official, asked about the possibility of extraterrestrial explanations for the observations, said: "That's not the purpose of the task force, to evaluate any sort of search for extraterrestrial life. That's not what we were charged with doing."
"Of the 144 reports we are dealing with here, we have no clear indications that there is any non-terrestrial explanation for them - but we will go wherever the data takes us," the official added.
All but one of them, an instance attributed to "airborne clutter," remain unexplained, subject to further analysis, US officials told reporters during a briefing describing the report's findings.
For the remaining 143 cases, the government has yet to rule in or out whether the sightings might be of extraterrestrial origin, the officials said.
Likewise, the task force lacks enough evidence to conclude whether any of those incidents represent some exotic aerial system developed either by a US government or commercial entity, or by a foreign power, according to the officials.
"Of data we have, we don't have any clear indications that any of these unidentified aerial phenomena are part of a foreign (intelligence) collection program, and we don't have any clear data that is indicative of a major technological advancement by a potential adversary," the senior official said.
"We continue to put a lot of effort and energy into tracking those types of developments, and we watch that very carefully."
"Nothing in this data set clearly points us into that direction," the official added.
The government in recent years has adopted UAP as its preferred term for what are otherwise known as "unidentified flying objects", or UFOs, long associated with the notion of alien spacecraft.
A second senior official said that 21 of the reports show UAPs "that appear to have some sort of advanced propulsion or advanced technology," and appear to lack any means of propulsion or acceleration and exhibit speeds beyond what the United States believes foreign adversaries possess.
The report was ordered by Congress as part of broader intelligence legislation signed by former President Donald Trump in December. Senator Marco Rubio was instrumental in commissioning it.
The report marks a turning point for the US government after the military spent decades deflecting, debunking and discrediting observations of unidentified flying objects and "flying saucers" backing back to the 1940s.
