Washington: In an unusual declaration, the US has said it has no evidence that extra-terrestrials exist and they have ever tried to contact humans.

"The US government has no evidence that any life exists outside our planet, or that an extra-terrestrial presence has contacted or engaged any member of the human race," wrote Phil Larson of the White House Office of Science & Technology Policy in a blog post on the White House website.

Larson made the unusual statement in response to petitions submitted through "We the People," a White House initiative that promises a response to any petition that can garner 5,000 signatures in 30 days, a media report said.

The successful petition had called on the White House to "formally acknowledge an extra-terrestrial presence engaging the human race."

Larson said that even though there is no proof that life exists outside Earth, "that doesn't mean the subject of life outside our planet isn't being discussed or explored."

Larson indicated that a host of US government agencies were working "toward the goal of understanding if life can or does exist off Earth," including the Mars Science Laboratory, SETI, the Search for Extra-terrestrial Intelligence and Kepler, the NASA spacecraft.