A former US military intelligence official has claimed there is evidence that could suggest aliens have reached Earth.

Key points: The US Defence Department funnelled $28.7 million into program investigating UFOs

The US Defence Department funnelled $28.7 million into program investigating UFOs Footage captured by US fighter jet shows "rotating" aircraft

Footage captured by US fighter jet shows "rotating" aircraft Former military intelligence official resigns because of secrecy around the program

Luis Elizondo, a former Pentagon official who recently led a government program to research potential UFOs, believes there is secrecy surrounding the program and told CNN, "There is very compelling evidence that we may not be alone".

Mr Elizondo led the Advanced Aviation Threat Identification Program from 2007 to 2012, which investigated UFOs. Parts of the program's findings still remain classified.

He resigned in October to protest against what he believed was excessive secrecy around the program.

Despite the program officially ending in 2012, The New York Times reports the investigation is ongoing and Mr Elizondo has a successor continuing his research, but he refuses to name them.

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Mr Elizondo suggested to CNN on Monday there was evidence that did not rule out the chance aliens had reached Earth, but said he could not speak on behalf of the Government.

He claimed the origin of aircrafts investigated as part of the research were difficult to identify.

"These aircraft — we'll call them aircraft — are displaying characteristics that are not currently within the US inventory nor in any foreign inventory that we are aware of," he said.

"These aircraft don't have any obvious flight services, any obvious forms of propulsion, and are manoeuvring in ways that include extreme manoeuvrability beyond, I would submit, the healthy G-forces of a human or anything biological.

"Hypersonic velocities, low observability, positive lift. Again, seemingly defying the laws of aerodynamics."

The former military intelligence officer also claimed during his investigation, "We found a lot".

He said however they did not have all the information and were filling in the gaps.

"And there is still, by the way, a lot that we don't really know," he said.

Sorry, this video has expired US fighter jet chases UFO

The US Defence Department revealed this week it spent $28.7 million on the program set up to investigate UFOs.

Its initial funding came largely at the request of former Democratic senator Harry Reid — the Nevada politician long known for his enthusiasm for space phenomena.

The New York Times reported most of the money went to a space research company run by billionaire Robert Bigelow, who is convinced aliens exist.

Footage of UFOs investigated showed US fighter jets chasing a fast moving aircraft off the coast of San Diego in 2004.

The footage was captured by a US Navy F/A-18 Super Hornet and pilots on board can be heard describing what they are witnessing.

"It's a f***ing drone, bro," a pilot is heard saying.

The pilots notice the aircraft is flying against the wind and claim it's rotating.

Following the release of the footage, one of the pilots, Commander David Fravor, told The New York Times: "It accelerated like nothing I've ever seen."

CNN reports Mr Reid was proud of the program and its ground-breaking studies.

ABC/Reuters