In a new sign of tensions with the United States, Iran on Tuesday reimprisoned the oldest American known to be held in that country, despite advice from its own doctors to extend his temporary leave because of potentially fatal heart problems.

The American, Baquer Namazi, 81, a dual citizen and former Unicef diplomat, had been given an emergency reprieve from Tehran’s Evin Prison on Jan. 28 because of worsening health.

That step had raised expectations that Mr. Namazi might be given a long-term medical parole, signaling Iran’s possible willingness to engage with the United States over prisoners, one of the festering issues in their estranged relationship.

Such expectations appeared dashed, however, with the news conveyed by the Namazi family’s American lawyer, Jared Genser, that Mr. Namazi had been abruptly ordered returned to prison on Tuesday.

In a telephone news conference from Washington, Mr. Genser said the order had overruled a recommendation by the Iranian government’s medical examiners, who had concluded that Mr. Namazi needed at least three months of leave.

“Iran’s decision against the advice of its own doctors to send Baquer Namazi back to Evin Prison is a shocking development and is tantamount to a death sentence that will be imposed quickly,” Mr. Genser said.

Mr. Namazi and one of his sons, Siamak, also a dual citizen of Iran and the United States, were convicted in October 2016 by an Iranian revolutionary court of collaborating with a hostile power — meaning the United States — after a closed trial. Their convictions and 10-year sentences were upheld last November on appeal. The precise nature of the accusations against them has never been disclosed.

Mr. Genser and the family’s other son, Babak, have described the Namazis as hostages and have asserted their innocence. The United States has called on Iran to release the Namazis and at least two other American citizens known to be held in Iranian prisons. The United Nations also has sought to intervene in the Namazi case, saying the father and son had been imprisoned arbitrarily and should be freed.

The elder Namazi has been hospitalized at least four times over the past year, Mr. Genser said, and doctors installed a pacemaker last September.

Babak Namazi, who joined Mr. Genser for the news conference, said he was “stunned and horrified that my father has been sent back to Evin Prison.”

There was no immediate comment from the Iranian government on Baquer Namazi’s reimprisonment.

In Washington, the under secretary of state, Steve Goldstein, told reporters that “we are deeply disturbed that the Iranian government has returned Mr. Namazi to prison,” and that Mr. Namazi remained “in urgent need of sustained medical care.” Asked whether he thought Mr. Namazi could die in prison, he said: “We are concerned about that, and we would hope the Iranians would be concerned about that, too.”

Unicef said in a statement that it was “deeply concerned about Mr. Namazi’s deteriorating health and continues to urge the Iranian government to grant him a full and unconditional release on humanitarian grounds.”

Jason Rezaian, the Washington Post correspondent who was among a group of American prisoners freed by Iran when the Iranian nuclear agreement took effect in January 2016, was sharply critical of Iran’s behavior in the Namazi case.

“I would call this news ‘shocking’ but I know the forces inside Iran responsible for Baquer Namazi’s detention too well,” Mr. Rezaian wrote on Twitter.

Iranian officials have consistently denied that they arbitrarily arrest Americans. They also have long contended that the United States unfairly imprisons Iranians for violating what Iran considers specious American sanctions against the country.

Perhaps coincidentally, the news that Baquer Namazi had been denied a three-month medical leave came a few days after an Iranian-American dual citizen in the United States, Ahmad Sheikhzadeh, received a three-month prison term in Brooklyn federal court for filing false tax returns and conspiring to evade financial sanctions on Iran.

Mr. Sheikhzadeh, 61, a former consultant to the Iranian Mission to the United Nations, had admitted wrongdoing but called the sentence “very unfair” and said he had been singled out for prosecution because he refused to act as an American informant.

On Monday, Iran’s Foreign Ministry spokesman, Bahram Qasemi, said at a news conference in Tehran that the verdict against Mr. Sheikhzadeh had been “politically motivated as the Americans are constantly engaging in psychological warfare against Iran.”

Other recent developments concerning the prisoner issue between the countries have suggested that it will not be resolved anytime soon, especially concerning dual citizens. Iran does not recognize American citizenship held by Iranians.

Last week, the Center for Human Rights in Iran, a New York-based advocacy group, reported that an Iranian revolutionary court had sentenced an Iranian-American art dealer, Karan Vafadari, and his Iranian wife, Afarin Neyssari, to long prison terms for unspecified activities that had aroused suspicion by the Islamic Revolutionary Guards.

The center suggested that both defendants had been singled out because they were Zoroastrian, an ancient religion in Iran whose followers have been persecuted since the 1979 revolution.