From the summer of 2014 to March 2015, US Navy pilots had reported about seeing strange objects in the sky, one of them like a spinning top moving against the wind, which appeared almost daily over the East Coast, a media reports said.
In this period, the pilots reported to their superiors that the objects had no visible engine or infrared exhaust plumes, but that they could reach 30,000 feet and hypersonic speeds, The New York Times said in the report published on Sunday.
"These things would be out there all day," Lt. Ryan Graves, an F/A-18 Super Hornet pilot who has been with the Navy for 10 years, and who reported his sightings to the Pentagon and Congress, told the newspaper.
"Keeping an aircraft in the air requires a significant amount of energy. With the speeds we observed, 12 hours in the air is 11 hours longer than we'd expect."
In late 2014, a Super Hornet pilot had a near collision with one of the objects, and an official mishap report was filed.
Some of the incidents were videotaped, including one taken by a plane's camera in early 2015 that shows an object zooming over the ocean waves.
Lieutenant Graves and four other Navy pilots, told The New York Times that they saw the objects in 2014 and 2015 in training manoeuvers from Virginia to Florida of the aircraft carrier Theodore Roosevelt.
The objects have gotten the attention of the Navy, which earlier this year sent out new classified guidance for how to report what the military calls unexplained aerial phenomena or unidentified flying objects.
The sightings were reported to the Pentagon's little-known Advanced Aerospace Threat Identification Program, which analyzed the radar data, video footage and accounts provided by senior officers from the Roosevelt.
Luis Elizondo, a military intelligence official who ran the programme until he resigned in 2017, called the sightings "a striking series of incidents".