US Undersecretary of State Steve Goldstein told reporters Baquer Namazi's health is failing and called for the former UNICEF official to be released immediately on humanitarian grounds.
"We are deeply disappointed that the Iranian government has returned Mr Namazi to prison. We are concerned about his declining health and he remains in urgent need of sustained medical care," Goldstein said.
On Sunday, Iranian authorities granted Namazi, a dual US-Iranian citizen, leave from prison on humanitarian grounds. In September, he underwent emergency heart surgery to install a pacemaker.
The Namazis were given 10 years in prison for "espionage and collaboration with the American government" in October, a charge denied by the family and dismissed by US authorities.
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"Mr Namazi is quite ill. He needs to get appropriate medical care very soon, and should be released on humanitarian grounds," Goldstein said.
Asked if Baquer Namazi might die, Goldstein said: "We are concerned about that and we would hope the Iranians are concerned about that too.
"We also renew our call for the return of all unjustly detained and missing citizens in Iran, including Baquer's son Siamak, Xiyue Wang and Robert Levinson," he added.
"We have raised this issue at every opportunity and will continue to do so, until their cases are resolved."
Baquer Namazi was rushed to a hospital on January 15 -- the fourth time in the last year -- after a "severe" drop in his blood pressure, and irregular heartbeat, Genser said on Sunday.
Wang, a Chinese-American researcher from Princeton University, has also been given a 10-year sentence for espionage.
Washington and Tehran have had no diplomatic relations since April 1980 in the wake of the Islamic revolution, and tensions have sharpened under Trump after a brief warming under his predecessor, Barack Obama.
The two countries along with other major powers signed an accord July 14, 2015 aimed at curbing Iran's nuclear program, but Trump's administration is pushing for tougher measures.
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