The State Department said they are "aware of reports" that Michael R. White has been in custody since late July.
A Navy veteran from California has been in custody in Iran for more than five months, his family said Tuesday.
Michael R. White was captured by authorities in late July, when he was there to visit his Iranian girlfriend, the vet's mom, Joanne White, told NBC News.
The State Department is "aware of reports" of White's detainment but declined further comment, officials said. It was not immediately known how long White had been out of the Navy.
Joanne White said she didn't want to discuss her son's detainment in detail for fear of putting him in any more danger.
"I don't want to say anything else at this time," she said.
White, 45, is from Imperial Beach, in the furthest southwest corner of San Diego County and the continental United States.
Back in 2014, White reached out to the American Rogues, a Celtic band dedicated to raising money for veterans, to ask for their help in raising $5,000 so that he could help a woman suffering from Lupus in the Middle East. It's not clear if the girlfriend White was visiting last year is this same woman from 2014.
"He said he was hoping to bring her to the U.S. He also stated he planned on repaying all donors," vocalist and band leader Nelson Stewart told NBC News in an email on Tuesday.
"I discussed the matter with my friends and advisors and we agreed that the proposal didn't fit the mandate of the band, which is to assist veterans directly. I wished Michael the best of luck and we never heard from him again."
The United States hasn't had diplomatic relations with the country since the overthrow of the shah in 1979. The Swiss government has acted as the intermediary for any talks between Washington D.C. and Tehran.
"All I know is that he is alive and they were putting in a request for a consular visit by the Swiss," Joanne White told The New York Times, adding that he was supposed to return from Iran on a flight a through Dubai.
White's detainment was first reported by Iranwire, an online news operation operated by Iranian expats, on Monday before it was picked up by The New York Times on Monday.
The Iranwire report was based on a former Iranian prisoner who had allegedly met White while they were both at Vakilabad Prison in the city of Mashhad in October, according to the Times.
White has been undergoing chemotherapy and radiation treatment for a neck tumor, his family said. He also suffers from acute asthma, according to his mother.
The American has been to Iran "five or six times" to visit his girlfriend there, Joanne White said.
Tensions between Iran and the U.S. have increased in the Trump administration after the president withdrew from the nuclear accords negotiated by the Obama White House.