The father of whistleblower Edward Snowden has said he is reasonably confident his son would return to the US if certain conditions were met.

In an interview with NBC television on Friday, Lonnie Snowden acknowledged that his son, a former National Security Agency contractor, broke the law by leaking about US surveillance programme, but did not think he committed treason.

"If folks want to classify him as a traitor, in fact, he has betrayed his government. But I don't believe that he's betrayed the people of the United States," he told NBC television's "Today" show.

Snowden senior said his attorney had informed Attorney General Eric Holder in a letter that he believed his son would voluntarily return to the US if the Justice Department promised not to hold him before trial and not subject him to a gag order, according to the US network.

Representatives for the Justice Department could not be reached immediately for comment on the letter.

The elder Snowden has not spoken to his son since April, but said he believed he was being manipulated by people at the whistleblowing website WikiLeaks, which has been trying to help Edward Snowden gain asylum.

"I don't want to put him in peril, but I am concerned about those who surround him," Lonnie Snowden told NBC.

"I think WikiLeaks, if you've looked at past history, you know, their focus isn't necessarily the Constitution of the United States. It's simply to release as much information as possible."

Snowden, who requested political asylum in Ecuador, has not been seen since he arrived in Moscow from Hong Kong on Sunday. Russian officials said he was in a transit area at Moscow’s Sheremetyevo airport.

Meanwhile, more than a quarter of the Senate's members asked the top US intelligence official on Friday to release more information on the government's bulk collection of data on Americans' communications.

The release of the material has prompted a firestorm of concerns about the extent of government data tracking as well as questions about lawmakers' role in approving the legislation that allowed the data collection.

Critics of the surveillance programmes see them as infringing on Americans' privacy rights, while backers said they are important tools for national security and subject to close control by the courts.