London (CNN) -- The lawyer for Amir Hekmati, an American sentenced to death in Iran for espionage, has called for the ex-Marine's case to be judged on humanitarian, not political grounds.
Pierre Prosper, who has been engaged by the 28-year-old's family to try to secure his release and return home, said Hekmati was at risk of falling victim to the animosity between the United States and Iran.
"What worries me most is that this case is entrapped in an intense political environment," the U.S.-based lawyer told CNN.
"We want to remove it from the political environment and establish a humanitarian corridor of communication and see if we can just talk about Amir as a human being."
Hekmati, who served in the U.S. Marines from 2001 to 2005, was arrested in August 2011 while visiting his grandmother and other relatives in Iran. His family has said he made the trip after obtaining permission from the Iranian Interests Section of the Pakistani Embassy in Washington.
On Monday, Iran's semi-official Fars news agency reported that he had been convicted of espionage, "working for an enemy country," being a member of the CIA and "efforts to accuse Iran of involvement in terrorism."
He has 20 days in which to appeal against his death sentence.
His family, who live in Michigan, claim that a "confession" aired on Iranian state television last month was coerced.
"We believe that there's a fundamental misunderstanding, and that the accusations against him are false," said Prosper.
"That said, we're prepared to open a line of communication with the government. We hope that they will accept it, because we believe that there is a workable way around this."
Switzerland, which represents U.S. interests in Iran, has officially confirmed the verdict and sentence, U.S. State Department spokesman Victoria Nuland said Tuesday. The United States has not had diplomatic relations with Iran since 1980, but "strongly condemns" the sentence and passed that condemnation to Iran through the Swiss, she said.
"We maintain, as we have from the beginning, that these charges against him are a fabrication and we call on the Iranian authorities to release him immediately," Nuland said. "We've also called on them to allow him to have legal counsel. Defendants in Iran are allowed to appeal within 20 days."
Nuland said Washington has urged Iranian authorities to allow Swiss diplomats to visit Hekmati, a step they have so far denied.
Prosper is a former ambassador-at-large for war crimes issues under the George W. Bush administration, and served as a war crimes prosecutor at the U.N. International Criminal Tribunal for Rwanda.
The high-profile lawyer said he hoped his previous experiences in Iran would help in the case.
"I've established some relationships based on my prior negotiations with the Taghavi case. I'll try to revive those connections and see if they are willing to have a conversation.
"It will be difficult, but it's possible, and one of the ways to do it is to move the government out of this, and have it be a communication from the family to the [Iranian] government."
Prosper said he had been shocked at the speed of the proceedings against Hekmati, and was concerned at a lack of openness surrounding the trial.
"This was literally a half-day trial; he was in detention only for a few months, and the verdict came within weeks. We also are troubled by the fact there's been no transparency, so it is really hard to see what happened."
Hekmati's family is "obviously shocked and troubled" at news he has been handed a death sentence, Prosper told CNN, particularly since they have had no contact with him in recent months.
"The family in the United States has not spoken with him since his incarceration, his detention. From the videos we've seen he looks to have lost weight, he appears to be under duress, and we're concerned.
"It is very difficult. I hear from them easily five or six times a day -- as you can imagine, the stress level is very high," he said.
"The news is not positive, but we try to reassure them that it is not over: We will engage with the government and we hope that they will show compassion.
"Our hope is that they will talk to us. Our hope is that they will listen to the family and... make a humanitarian decision.
"The family's message [to Iran] is: Please show some compassion. You've made your decision, we obviously disagree with the decision, but we are where we are, please allow him to come home, back to the United States."
CNN's Elise Labott contributed to this report.