Iran has confirmed it is holding a US Navy veteran – a development likely to worsen relations between the two countries even further.
Michael White, 46, was arrested in July while visiting his Iranian girlfriend and is being held on unspecified charges, the New York Times reported earlier this week.
His mother, Joanne, said she reported his disappearance after he failed to return to his home in Imperial Beach, California, but the State Department told her only three weeks ago that he was being held in Iran.
He had booked a return flight to San Diego for 27 July, she said.
Mr White, who has acute asthma and has been treated for a tumour in his neck, has been to Iran to see his girlfriend five or six times, she added.
Iran's foreign ministry has not specified when he was arrested, or what he is accused of.
But foreign ministry spokesman Bahram Qassemi said Iran had notified the US government of White's arrest within days of it happening.
He added that Mr White had been arrested "some time ago", according to the semi-official Tasnim news agency, which is believed to be close to the country's paramilitary Revolutionary Guard.
London-based news website IranWire said Mr White was being held in Vakilabad prison in Mashhad, northeastern Iran.
That was according to a former detainee, Ivar Farhadi, who said Mr White's life was in danger because the jail contained dangerous inmates.
Reports that Mr White had been poorly treated were denied by Mr Qassemi.
"These sorts of reports are lies and improper and are seriously rejected," state news agency IRNA quoted him as saying.
Relations between the US and Iran have been strained since President Donald Trump withdrew from an international agreement on Iran's nuclear programme.
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He also re-imposed sanctions last year that damaged Iran's economy.
Several US citizens have been held in Iran in recent years, with Mr Trump warning in 2017 that Tehran would face "new and serious consequences" unless all unjustly held American citizens were freed.