A Pentagon report says it can only explain one of 144 sightings made by the US military since 2004.
The US government has said it has no explanation for dozens of unidentified flying objects seen by military pilots.
A Pentagon report released on Friday says of 144 reports made about the phenomena since 2004, all but one remain unexplained.
It does not rule out the possibility that the objects are extraterrestrial.
Congress demanded the report after the US military reported numerous instance of objects seen moving erratically in the sky.
The interim report released on Friday said most of the 144 reported cases of the “unidentified aerial phenomena”, referred to as UAP, came in the last two years, after the US Navy put in place a standardised reporting mechanism.
In 143 of the reported cases, they “lack sufficient information in our dataset to attribute incidents to specific explanations”.
UAP “probably lack a single explanation”, the report said. Some could be technologies from another nation like China or Russia, others could be natural atmospheric phenomena like ice crystal that could register on radar systems, while the report also suggested some could be “attributable to developments and classified programs by US entities”.
The one case they could identify “with high confidence” was identified as “a large, deflating balloon”, the report said.
It added that the UAP pose “a clear safety of flight issue and may pose a challenge to US national security”.