"We're going to make a very strong case between now and the end of this administration that this is one of those rare cases for which the pardon power exists," Ben Wizner, the ACLU lawyer who is Snowden's principal legal adviser, told New York magazine in a cover story published late on Sunday.

"It's not for when somebody didn't break the law. It's for when they did and there are extraordinary reasons for not enforcing the law against the person."

Snowden, however, conceded that Obama is unlikely to offer such a pardon before he leaves office.

"There is an element of absurdity to it," he said. "More and more, we see the criticisms levelled toward this effort are really more about indignation than they are about concern for real harm."