Russia could return Edward Snowden to the US as a "gift" to President Donald Trump, according to two US intelligence sources cited by NBC News on Friday.

One unnamed official, who NBC said gleaned information from "a series of highly sensitive intelligence reports," said such a move could be an attempt to "curry favor" with the Trump administration.

Snowden is a former US National Security Agency contractor who stole top-secret documents in 2013 that revealed mass surveillance efforts by the US government. He shared those documents with journalists. Russia has been sheltering Snowden since 2013, and recently granted him permission to stay through 2020.

Trump has in the past called Snowden a traitor and a spy, and suggested Snowden may have given US secrets to other countries. Snowden denied those allegations on Friday, saying on Twitter, "I never cooperated with Russian intel. No country trades away spies, as the rest would fear they're next."

Snowden also tweeted an interview he gave to Yahoo News' Katie Couric, in which he again declared, "I'm independent ... I have always worked on behalf of the United States ... Russia doesn't own me."

The notion that Russia could send Snowden back to the US as a gift to Trump is buoyed in part by Trump's stated desire for warmer relations with the Kremlin.

Trump, throughout the presidential election, expressed an affinity for Russian President Vladimir Putin. Those intimations were often echoed by Trump surrogates, including national security adviser Michael Flynn.

Those moves were regularly condemned by both Republican and Democratic leaders who have warned that Putin cannot be trusted.

In an interview with NBC News, former deputy national security adviser Juan Zarate said, "I think at the end of the day, Moscow holds the cards here."