Ryan Graves said the number of unidentified flying objects, or UAPs (unidentified anomalous phenomena), was being "grossly underreported" and was an "urgent" issue.

"Military air crew and commercial pilots are frequently witnessing these phenomenons," he told a committee hearing.

"The stigma is real and powerful and challenges national security."

Mr Graves was giving evidence at a session held by the House Oversight Subcommittee, which covers US national security and foreign affairs.

The hearing - the first of its kind - was announced last week amid increased attention on reported UFO sightings.

The committee's Republican chairman Glenn Grothman opened the session by saying a "lack of transparency" had fuelled "wild speculation" that had eroded "public trust" in government institutions.

Democrat member Robert Garcia said stories of UFOs and UAPs "warrant investigation and oversight".

Mr Graves said he first began detecting "unknown objects" in US airspace in 2014, while stationed at Virginia Beach.

He said during a training mission 10 miles off the coast, two aircraft got "split" by a UAP described as a "dark grey or black cube inside a clear sphere".

Despite raising safety concerns, no official log of the incident was made, he claimed.

He said such "excessive classification practices" over decades were keeping "crucial information hidden".

"If they are foreign drones, it is an urgent national security problem," he said.

"If it is something else, it's an issue for science. In either case, they are an issue for flight safety."

A report from the Pentagon in 2021 found no evidence linking UAPs to extraterrestrials, with most of the sightings believed to be drones or balloons.

An example from this year was a Chinese "spy balloon" that was shot down over US airspace.

Mr Graves was one of three men called to give evidence on Wednesday.

The other two were ex-Navy commander David Fravor and former intelligence official David Grusch.

Mr Fravor has also said to have seen UAPs, while Mr Grusch has claimed the US has retrieved "intact and partially intact" vehicles of nonhuman origin.

Mr Grusch told the hearing he became a whistleblower after receiving reports from "esteemed" current and former military officials that the US government was hiding information about UAPs.

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Joe Biden's administration has faced calls to declassify documents related to UFOs, and this month the Senate introduced an amendment to annual defence policy legislation that seeks to do just that.

Interest has been so been high that NASA held its first public meeting on the issue earlier this year.

A panel of scientists and retired astronauts said there had been a spike in sightings, though only a small fraction contained signals which could be deemed "anomalous".

The US space agency started referring to UFOs as UAPs because of how the term is used in pop culture.

Dr Daniel Evans, NASA's assistant deputy associate administrator for research, told the meeting in May that the agency's preferred acronym reflected the topic was "serious business".