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 Evin Prison in Tehran 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.