The US State Department said on Tuesday it was "aware of reports" of his detention but declined to release details, citing privacy considerations.
London-based news website IranWire, which first reported Mr White's arrest, quoted a former detainee, Ivar Farhadi, as saying he had been held at the same prison, Vakilabad, in Mashhad, northeastern Iran. On the third trip, he was arrested at an worldwide airport in Mashad and has been in prison since the summer of 2018.
Mr Farhadi told IranWire that Mr White's life was in danger because he was being kept in a prison with unsafe criminals.
White spoke about having what Farhadi called a gland that White suspected of being cancerous and "didn't seem to have any medical and nutritional services at all", Farhadi said.More news: Djokovic thrashes Murray in Open practice match
Michael White flew from Los Angeles to Dubai on July 9, and then on to Iran, his mother said. But he did not see any bruises or cuts on the American.
"According what he said to me, he was denied the right to use [the] telephone and visits and access to a lawyer". The family of Michael White revealed this week that he's been in captivity since last summer and his mother said she has no idea about any possible charges against him.
United States officials have also pushed Iran for information about the whereabouts of Robert Levinson, a former Federal Bureau of Investigation agent and Central Intelligence Agency contractor who went missing in Iran in 2007.
Iran confirmed on Wednesday it had arrested an American, confirming U.S. media reports about a case that risks further worsening relations with Washington.More news: Gordon Ramsay faces Twitter wrath over sexual innuendos toward Sofia Vergara
He had booked a return flight to San Diego for 27 July, she said. She filed a missing-person report and had been communicating with the State Department and Department of Homeland Security.
"I'm very anxious about his health", she said, according to CBS News.
At least three other American citizens, two of whom are of Iranian descent, have been under arrest in Iran for years while another American has been missing for more than a decade. "I promised to myself that if I got out of prison one of the first things that I would do would be to inform people about the situation of this American prisoner".
Siamak Namazi, and his father, Baquer Namazi, are Iranian-Americans who have been detained since October 2015 and February 2016, respectively.More news: Meghan Markle and Prince Harry's $4.5m temporary farm home
United Nations human rights experts said both father and son were detained arbitrarily.