"We are aware of reports of the detention of a United States citizen in Iran".
Mr Qassemi said Iran had informed the USA government about Mr White's arrest within days of when he was taken into custody and denied that he had been poorly treated. 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.
Iran had not confirmed the arrest before the state news agency IRNA carried a statement by foreign ministry spokesman Bahram Qassemi on Wednesday, which did not specify when it had happened or what crime he was accused of. He met her online about four or five years ago, and he had visited her four times over the last few years, Joanne White said.
White's imprisonment could further worsen relations between Washington and Tehran, longtime foes. She said she filed a missing person report after he failed to return to work in California in late July.
Qassemi said, "We strongly reject such untrue and false reports".More news: Chelsea Closing In On Gonzalo Higuain
"We have no higher priority than the safety and security of U.S. citizens overseas".
She is anxious about her 46-year-old son's health because he has had cancer and suffers from asthma, she said. "Due to privacy considerations, we have no additional information to provide at this time".
Since October 2015, it has also been holding Iranian-American Siamak Namazi.
Iran has confirmed a United States media report that a United States military veteran was arrested past year, but denied he had been badly treated.
New details are emerging about an American Navy veteran being held captive in Iran.More news: Australian police arrest man over suspicious packages sent to embassies and consulates
In January 2016, Iran freed four USA prisoners - Washington Post journalist Jason Rezaian, Marine veteran Amir Hekmati, Christian pastor Saeed Abedini and Nosratollah Khosravi-Roodsari - in exchange for the U.S. granting clemency to seven Iranians indicted or imprisoned in the United States.
The first Iranian arrest of an American under the Trump administration has been confirmed by Iran, heightening already taut relations between the two countries.
President Donald Trump withdrew from an global agreement Iran's nuclear programme and re-imposed sanctions a year ago that hit the Iranian economy hard.
Speaking about the relations between countries, Iran leader Ayatollah Ali Khamenei called US officials "first-class idiots" Wednesday.More news: French ministers aghast at support for boxer who fought police