In the US, The Pentagon has confirmed it had a UFO investigation programme, but says it was ended in 2012 to fund "higher priority issues".
The New York Times reported the secret Advanced Aviation Threat Identification Programme, which was tasked with investigating sightings of unidentified flying objects, ran from 2007 to 2012 with US$22m (€18.7m) in annual funding.
The newspaper said the programme produced documents that describes "sightings of aircraft that seemed to move at very high velocities with no visible signs of propulsion, or that hovered with no apparent means of lift".
The programme's existence was not classified but operated with the knowledge of an extremely limited number of officials, the report said.
It also said initial funding came in large part because former Senate Democratic leader Harry Reid had a passion and enthusiasm for space phenomena.
Mr Reid posted a link to the New York Times article on Twitter, writing: "The truth is out there. Seriously."
Also tweeting a link to the article, Mr Reid's former spokeswoman Kristen Orthman said: "If you've talked to Harry Reid for > 60 seconds then it's the least surprising thing ever that he loves UFOs and got an earmark to study them."
If you’ve talked to Harry Reid for > 60 seconds then it’s the least surprising thing ever that he loves UFOs and got an earmark to study them. https://t.co/GoMJx4X43v
— Kristen Orthman (@KristenOrthman) December 16, 2017
Following the article's publication, Pentagon spokeswoman Laura Ochoa told Reuters: "The Advanced Aviation Threat Identification Programme ended in the 2012 timeframe.
"It was determined that there were other, higher priority issues that merited funding and it was in the best interest of the DoD to make a change."
However, the US Department of Defence was unclear on whether the so-called Advanced Aviation Threat Identification Programme still operates in some capacity.
Ms Ochoa said: "The DoD takes seriously all threats and potential threats to our people, our assets, and our mission and takes action whenever credible information is developed."