Iran’s penal authorities granted a four-day leave on Sunday to Baquer Namazi, the oldest of the United States citizens known to be imprisoned in Iran, because of his worsening health, Mr. Namazi’s American lawyer said.
The announcement by the lawyer, Jared Genser, raised speculation that the reprieve was a precursor to parole for Mr. Namazi, 81, a dual Iranian-American citizen who was formerly a diplomat for Unicef. Mr. Namazi’s son, Siamak Namazi, 45, also a dual citizen, remains imprisoned.
Parole for the elder Mr. Namazi would be the first serious sign of movement on Iran’s part concerning the Americans known to be incarcerated there since a group was released when the Iranian nuclear agreement took effect two and a half years ago.
The prisoners have long been a sore point in the estranged relations between the United States and Iran, which have worsened under the Trump administration. President Trump has threatened to scrap the nuclear accord, which lifted many economic sanctions on Iran in exchange for verifiable reductions in its nuclear activities.
The Namazis were convicted of collaborating with a hostile power — the United States — in a secretive trial in Iran in October 2016, and their convictions and 10-year sentences were upheld last November on appeal. The precise nature of the accusations has never been made clear.
The men have been confined in different wards of Evin Prison, a notorious penitentiary in Tehran, the capital, and the father has suffered worsening health problems — including low blood pressure and an irregular heartbeat. He was transferred to a hospital four times in the past year, Mr. Genser said, and had a pacemaker installed last September.
Mr. Genser said that under the terms of his temporary release, Mr. Namazi was allowed to rest at his home in Tehran, but that “he cannot leave Iran and is prohibited from speaking to the media.”
Mr. Namazi is supposed to be returned to prison on Wednesday. But the lawyer said that the father’s Iranian doctors had “strongly advised against his return to Evin Prison” because the conditions would “rapidly exacerbate his ailing health.”
There was no immediate comment from the Iranian government. United Nations and United States officials greeted the news of Mr. Namazi’s temporarily release with lukewarm approval.
Najwa Mekki, a Unicef spokeswoman, said in an emailed statement “While we welcome this development, we are concerned that it is only temporary.” She said Unicef “continues to urge the Iranian government to grant him a full and unconditional release on humanitarian grounds.”
State Department officials were also guarded in their reaction.
“While this is a good start and we appreciate the gesture, to put him back into prison is wrong,” said Steve Goldstein, the State Department under secretary for public affairs. “He needs to be allowed to return to the United States to get proper medical care,” Mr. Goldstein said.
Mr. Goldstein said the State Department had been working to secure the release of the Namazis and others held in Iran. Thomas A. Shannon, the under secretary of state, raised the issue on the sidelines of a meeting last year between Iran and the six big powers that negotiated the nuclear accord.
Mr. Goldstein said that neither the United States nor Iran wanted to repeat what happened with Otto F. Warmbier, a University of Virginia student who fell into a coma while imprisoned in North Korea and died shortly after being sent home.
At least two other American citizens are known to be imprisoned in Iran: Xiyue Wang, a Princeton graduate student who was seized while working on his doctoral thesis, and Karan Vafadari, a Tehran art gallery owner. Another American, Robert Levinson, has been missing in Iran for more than 10 years.
Mr. Wang’s wife, Hua Qu, said in a telephone interview last week that she was able to speak with her husband once a day via a monitored phone at Evin Prison. She said that her husband was the only non-Iranian in a 20-inmate cellblock and that “it’s very miserable for him.”
The prisoners’ cases have been complicated by Iran’s refusal to recognize incarcerated Iranian-Americans as Americans. Iran also contends that the United States is holding a number of Iranian citizens on unjustified grounds.