The New York Times reported on Monday that Michael R. White, a 46-year-old US Navy veteran, was seized while visiting Iran and has been held in jail since July on unspecified charges.
His mother, Joanne White, told the New York Times her son had visited Iran "five or six times" to meet his Iranian girlfriend. He had bought a ticket to return from Iran, but never boarded his flight on July 27.
Asked about the reports, a US State Department spokesman said: "We are aware of reports of the detention of a US citizen in Iran."
The spokesman declined to provide additional information, citing privacy considerations.
Iranian officials have not reacted to the reports, and were not immediately available for comment.
A former detainee in Iran, Ivar Farhadi, told the London-based IranWire website he had spoken to White when they were both at Vakilabad Prison in the city of Mashhad in northeastern Iran.
The New York Times quoted White's mother as saying her son, a California resident, suffers acute asthma and had undergone chemotherapy and radiation treatment for a neck tumor.
Tension between Iran and the United States has risen significantly since last May, when US President Donald Trump withdrew from an international nuclear deal with Tehran and reimposed US sanctions that had been lifted after a 2015 accord.
Several Americans have been detained in Iran in recent years and Trump warned in 2017 that Tehran would face “new and serious consequences” unless all unjustly held US citizens were freed.
Former FBI agent Robert Levinson disappeared while visiting Iran’s Kish Island in 2007. US officials believe Levinson, who suffered from diabetes, died in captivity after meeting with an American-born Islamic militant. Iranian officials have repeatedly denied knowledge of his disappearance or whereabouts.
In October 2015, Siamak Namazi, a businessman in his mid-40s with dual US-Iranian citizenship, was detained as he was visiting family in Tehran. His 82-year-old father, Baquer Namazi, was also arrested in February 2016 and later convicted of espionage charges which he denied.Xiyue Wang, a Chinese-born US citizen and graduate student from Princeton University, was arrested in Iran in 2016. He was sentenced to 10 years in jail on spying charges that he denied.