The White House says it has no evidence that extraterrestrial creatures exist.
The Obama administration made the unusual declaration in response to a feature on its website that allows people to submit petitions that administration officials must respond to if enough people sign on.
In this case, more than 5,000 people signed a petition demanding that the White House disclose the government's knowledge of extraterrestrial beings.
More than 12,000 signed another petition seeking formal acknowledgement of an extraterrestrial presence engaging the human race.
In response, Phil Larson, of the White House Office of Science and Technology Policy, wrote that the US government has no evidence that life exists outside Earth, or that an extraterrestrial presence has contacted 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," Mr Larson wrote.
He did not close the door entirely, however, on a close encounter of an alien kind, noting that many scientists and mathematicians believe that, statistically speaking, odds are high that there is life somewhere among the "trillions and trillions of stars in the universe" - although odds that humans might make contact with non-humans are remote.
It is not the first petition to force the White House to engage on an offbeat topic since the "We the People" web page was inaugurated in September. The White House has been forced to explain why it cannot comment in response to a petition demanding "Try Casey Anthony in Federal Court for Lying to the FBI Investigators" - because it is a law enforcement matter.
Various petitions demanding legalisation of marijuana have gathered more than 100,000 names, to which the White House argues that marijuana is associated with addiction, respiratory disease and cognitive impairment, and legalising it would not be the answer.
The White House also has addressed topics including gay marriage and student loan debt.
When the website debuted, the White House promised to respond to any petition that offered 5,000 or more signatures within 30 days, but it now has now raised that threshold to 25,000.