David Grusch said he was "absolutely" certain the US government is in possession of non-human vehicles.

Asked at a Congress hearing on UFOs whether the "pilots who piloted this craft" were also found, he said "biologics came with some of the recoveries, yeah".

He was then asked if the biologics were "human or non-human".

"Non-human", he replied, adding: "That was the assessment of people with direct knowledge of the programme I talked to, that are currently still on the programme."

He said he prefers to use the term "non-human" rather than alien or extraterrestrial.

Mr Grusch was giving evidence at a hearing in which an ex-Navy pilot claimed the US government was "hiding crucial information" about UFOs.

The session was held by the House Oversight Committee, which covers US national security and foreign affairs.

The hearing was announced last week amid increased attention on reported sightings of UFOs - officially known as unidentified aerial phenomena or UAPs.

Mr Grusch, who said he has not personally spotted a UAP, told the panel that he knows of "multiple colleagues" who were injured by UAPs, citing interviews with 40 witnesses over a four-year period.

He said he led Defence Department efforts to analyse reported UAP sightings and was informed of a programme aimed at collecting and reconstructing crashed spacecraft.

This programme had been running for decades and was concealed by the US, he said.

Asked whether the US government had information about extraterrestrial life, Mr Grusch claimed it was likely the US had been aware of "non-human" activity since the 1930s.

The Pentagon has denied Mr Grusch's claims of a cover-up. In a statement, Defence Department spokeswoman Sue Gough said investigators have not discovered "any verifiable information to substantiate claims that any programmes regarding the possession or reverse-engineering of extraterrestrial materials have existed in the past or exist currently."

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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".