The U.S. government will not budge on its demand that former National Security Agency contractor Edward Snowden return to face prosecution for stealing thousands of classified intelligence documents, despite new calls for President Barack Obama to pardon him, U.S. officials said on Tuesday.

The officials said they expect Snowden's supporters to use the Thursday release of "Snowden" - directed by veteran filmmaker Oliver Stone - to mount a public campaign demanding a pardon before Obama leaves office in January.

Snowden, who lives in Moscow, is scheduled to appear via video link on Wednesday at a New York press conference, where advocates from human rights groups will call for a pardon.

They argue that Snowden performed a public service by exposing excessive and intrusive electronic spying by the intelligence agency and its English-speaking allies, including Britain's Government Communications Headquarters (GCHQ).

In an interview published by The Guardian on Tuesday, Snowden said the U.S. Congress, the courts and the president all "changed their policies" as a result of his disclosures, and that "there has never been any public evidence that any individual came to harm as a result."

White House spokesman Josh Earnest said on Monday that Snowden is charged with "serious crimes, and it's the policy of the administration that Mr. Snowden should return to the United States and face those charges."

Two other U.S. officials said there are no discussions inside the Justice Department about granting him a pardon.

Some officials have acknowledged that Snowden raised legitimate questions about the extent and effectiveness of some electronic eavesdropping, particularly the NSA's sweeping collection of "metadata" on domestic telephone calls by Americans, a practice that was curtailed after his revelations.

Other officials, however, say the material Snowden gave the media included sensitive details about the locations and operations of U.S. and allied global spying operations, some of which were compromised.