A former NASA engineer claims he saw a nine-foot alien while monitoring a space mission in 1991.

Clark McClelland was allegedly part of a team based at the Kennedy Space Center in Cape Canaveral, Florida, when he spotted the extra-terrestrial humanoid.

The 'entity', he says, stood on two legs and approached two U.S. astronauts, that he would not name, for one minute and seven seconds in the space shuttle bay.

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Sighting? Clark McClelland, allegedly pictured during his time with NASA, claims he saw a nine-foot alien

The entity, he claims in a YouTube video, was standing on two legs and interacted with two US astronauts

His story has been hailed by supporters as the most conclusive evidence that intelligent life exists in space.

McClelland claims the U.S. government has stripped him of his pension and forced him to live on social security, despite insisting he has served on more than 800 NASA missions.

But in a series of YouTube videos, he implores the public to take him seriously.

'I love my planet, I love our human race. I'm trying to serve all of you, if only you will let me,' he tells viewers.

Video: Stargate Chronicles

Getting his story out: McClelland has posted pictures from his time with NASA but claims nobody believes him

History: This is allegedly a picture of McClelland (left) with Judy Resnick (right), America's second female astronaut who died in 1986 when her Challenger shuttle broke apart 73 seconds into flight

After more than 800 missions with NASA, he says, he has no pension and lives off social security

McClelland's Twitter profile features a picture billed as his younger self in NASA uniform.

In a picture on his website he poses with NASA astronauts by a space shuttle.