A senior White House official has insisted there is "no credible evidence of extraterrestrial presence here on Earth".
And in a blow to millions of conspiracy theorists, the member of Barack Obama's administration added there was no credible information to suggest evidence of alien life is being kept from the public.
The unusual comments were in response to several petitions lodged on a White House website.
Senior space policy official Phil Larson said: "The US government has no evidence that any life exists outside our planet, or that an extraterrestrial presence has contacted or engaged any member of the human race.
"In addition, there is no credible information to suggest that any evidence is being hidden from the public's eye.
"Many scientists and mathematicians have ... come to the conclusion that the odds are pretty high that somewhere among the trillions and trillions of stars in the universe there is a planet other than ours that is home to life.
"Many have also noted, however, that the odds of us making contact with any of them - especially any intelligent ones - are extremely small, given the distances involved.
"But that's all statistics and speculation. The fact is we have no credible evidence of extraterrestrial presence here on Earth."
The statement was issued in response to two petitions, with a total of 17,000 signatures, calling on the Government to disclose knowledge of extraterrestrial beings and contact with them.
The White House has started a new initiative called "We the People" by allowing people to submit petitions to their government, and pledged to respond to some that get a significant following.