The National Security Agency leaker, Edward Snowden, would be willing to enter talks with attorney general Eric Holder to negotiate his return to the US, his legal adviser said on Sunday, but not without a guarantee of amnesty.

Jesselyn Radack said she was glad Holder indicated last week he would talk to lawyers for Snowden to negotiate his return from Russia, but said that he would need better protection.

“It’s a little disheartening that [Holder] seemed to take clemency and amnesty off the table, which are two of the negotiating points,” said Radack, who was interviewed via satellite from Moscow by NBC’s Meet the Press. “But again, none of us have been contacted yet about restarting negotiations.”

Holder said in an interview on MSNBC on Thursday that the US would not consider the idea of amnesty for Snowden “where we say, ‘No harm, no foul'”.

Radack, the director of national security and human rights at the Government Accountability Project – a whistleblowers’ organization – said Snowden, who leaked thousands of documents to media outlets including the Guardian, has already suffered because his US passport has been revoked.

“He has been punished quite a bit already and while we are glad to dialogue and negotiate, he is not going to come back and face an espionage prosecution,” she said.

On Thursday, in a website Q&A, Snowden discussed what conditions would be necessary if he were to return to the US.

“Returning to the US, I think, is the best resolution for the government, the public and myself, but it’s unfortunately not possible in the face of current whistleblower protection laws, which through a failure in law did not cover national security contractors like myself,” Snowden wrote.

Michael Chertoff, former secretary of homeland security under President George W Bush, appeared after Radack on Meet the Press. He said Snowden could get a fair trial if he returned to the country but was unlikely to be granted amnesty.

“If he decides he wants to tell the US government everything he stole he may be able to bargain for some kind of reduced sentence,” Chertoff said, adding that Snowden was potentially facing a life sentence, or 25-30 years in jail.

Chertoff added that the government has “done deals in the past with spies” – comparing Snowden to Robert Hanssen, a former FBI agent who spied for Soviet and Russian intelligence services from 1979 to 2001.

Radack said allegations that Snowden was a spy were unsubstantiated and the result of a smear campaign by the government. “Mr Snowden publicly chatted with the US this week to deny being a spy,” she said, “but if people don’t want to take my word for it, or Mr Snowden’s word for it, you can ask the FBI, which decided and still believes he acted alone.”

Also on Sunday, a German public broadcaster, ARD, broadcast an interview with Snowden.