Image caption Edward Snowden was last reported to be in transit at Moscow airport

The father of leaker Edward Snowden has said he believes his son would return to the US on certain conditions.

Lon Snowden asked for "ironclad assurances" his son's rights would be protected in a letter to Attorney General Eric Holder, seen by the BBC.

He asked his son not be held before trial nor subjected to a gag order, and be able to choose where he was tried.

Edward Snowden, who faces spy charges in the US, flew to Moscow last weekend and has requested asylum in Ecuador.

"Mr Snowden is reasonably confident that his son would voluntarily return to the United States if there were ironclad assurances that his constitutional rights would be honored," said the letter by Lon Snowden's lawyer, Bruce Fein.

Exploited by Wikileaks?

The correspondence also requested for the case against the former intelligence contractor to be dismissed in the event that any of the three conditions were not met.

Media playback is unsupported on your device Media caption The BBC's Steve Rosenberg explores the hotel Edward Snowden is believed to be in

Earlier on Friday, Lon Snowden told NBC News he had not spoken to his son since April, a month before he fled to Hong Kong after leaking to media details of a huge US snooping programme.

Mr Snowden said his son had broken the law, but denied he was a traitor.

"At this point I don't feel that he's committed treason," he said. "He has in fact broken US law, in a sense that he has released classified information."

He also voiced concern his son was being exploited by Wikileaks, which has offered legal assistance to the 30-year-old.

"I don't want to put him in peril, but I am concerned about those who surround him," he said.

"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."

Chinese 'misbehaviour'

Mr Snowden flew last Sunday from Hong Kong to Moscow's Sheremetyevo Airport, where Russian authorities say he remains in a transit zone.

On Friday, the diplomatic fallout from the affair continued as a US envoy accused China of "misbehaviour" for allowing Mr Snowden to leave.

Transit zones: A legal grey area Sheremetyevo airport transit area is Russian sovereign territory, but Russia says that in staying there, Snowden has not formally entered the country

Legally, an arriving air passenger may be said to "cross the border" only after clearing immigration, which would require an entry visa

If Russia allowed Mr Snowden to enter, it could implicate the state in helping a fugitive, analysts say How to travel without a passport

"I don't think we had a good-faith partner throughout that process," said Stephen Young, the US consul general in Hong Kong, warning of repercussions.

The city's government has said the US arrest paperwork had clerical errors, and that it had no legal basis to stop Mr Snowden travelling to Russia.

Beijing has accused the US of "double standards" on cybersecurity.

Ecuador says it has not yet processed the former US National Security Agency contractor's request for asylum.

Russian authorities complained on Friday the US had not informed them in time that Mr Snowden's passport had been revoked, placing Moscow in a "tough spot".

"If this fact had been known in advance, then possibly Mr Snowden might not have flown to Moscow and this entire story might never have happened," an unnamed Russian official told Interfax news agency.

Russian President Vladimir Putin this week refused to hand over Mr Snowden to Washington, saying he was a "free man".

Late on Thursday, Venezuelan President Nicolas Maduro repeated his offer of sanctuary to the US fugitive.