The Pentagon has provided a classified briefing to members of Congress about reported encounters by US navy pilots and unidentified aircraft, some of which were said to have no visible engines and could reach hypersonic speeds.

Earlier this year, a number of pilots reported seeing the objects on an almost daily basis from the summer of 2014 to March 2015, while flying navy jets off the East Coast. Some of the encounters were even captured on video, leading the navy to announce it had updated the way pilots were to formally reports the incidents.

Now, the military has provided a classified briefings on the encounters to a group of senators, after the president said last week he had been updated on the issue.

“Navy officials did indeed meet with interested congressional members and staffers on Wednesday to provide a classified brief on efforts to understand and identify these threats to the safety and security of our aviators,” the navy said in a statement, of the briefings that took place on Capitol Hill on Wednesday and Thursday.

“Navy officials will continue to keep interested congressional members and staff informed. Given the classified nature of these discussions, we will not comment on the specific information provided in these Hill briefings.

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Reports suggest the briefings, first detailed by Politico, were given to members of the Senate intelligence committee and their staff. Among those who had requested the update, was Democratic senator Mark Warner, vice chairman of the committee.

He was particularly interested, it appears because a number of the plots who reported the encounters had been flying out of Naval Air Station Oceana, a navy facility located in Virginia Beach, Virginia. They told the New York Times they had experienced the incidents along the coast from Virginia to Florida.

“These things would be out there all day,” Lt Ryan Graves, an F/A-18 Super Hornet pilot with ten years experience with the navy, 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.”

Another incident, that was captured by a plane’s camera in 2015, showed an object flying across the ocean as the pilots questioned what it was.

Donald Trump asked if he believes UFO sightings are real

“Wow, what is that, man,” one of them said.

“If pilots at Oceana or elsewhere are reporting flight hazards that interfere with training or put them in danger, then the senator wants answers,” said Mr Warner’s spokesperson, Rachel Cohen.

She added: “It doesn’t matter if it’s weather balloons, little green men, or something else entirely – we can’t ask our pilots to put their lives at risk unnecessarily.”

Last week, Mr Trump said he had already been briefed on the reported encounters. “I did have one very brief meeting on it,” the president told ABC News. “But people are saying they’re seeing UFOs – do I believe it? Not particularly.”

Interest in the reported encounters soared after it was revealed in 2017 the Pentagon had established a programme to study “unidentified aerial phenomenon” and interview pilots. It did so at the request of then sensor Harry Reid, of Nevada.

While the Pentagon says it is taking the reports seriously, it has not yet used the word “alien” to describe the incidents.