(CNN) US Navy veteran Michael White is in Iranian custody, the Islamic Republic's Foreign Ministry said Wednesday, confirming claims made by White's mother a day earlier.

"Michael White was arrested in the city of Mashhad a while ago, and within a few days after his arrest the US government was informed of the arrest through the Swiss Embassy in Tehran," ministry spokesman Bahram Ghasemi said.

In the absence of relations, the Swiss Embassy represents US diplomatic interests in Iran. The holy city of Mashhad is in northeastern Iran, 80 kilometers (about 50 miles) from its border with Turkmenistan.

Ghasemi denied allegations that White, who hails from Imperial Beach, California, has been mistreated in prison.

"This person's case is currently being processed by officials. The public will be informed of more details of the case in due time," he said.

Speaking to CNN on Tuesday, his mother, Joanne White, declined to discuss why her son was being held. She said she filed a missing person report after he failed to return to work in California in late July.

She is worried about her 46-year-old son's health because he has had cancer and suffers from asthma, she said.

"My heart aches for him. ... I just want to see him home safe and sound," Joanne White said by phone.

Visiting his girlfriend?

News about the American's detention comes amid rising tensions between the United States and Iran. Those strains have increased since May when the United States withdrew from a nuclear deal and reinstated sanctions against the country

A US State Department representative said the department was "aware of reports of the detention of a US citizen in Iran," and that it has "no higher priority than the safety and security of US citizens abroad."

"Due to privacy considerations, we have no additional information to provide at this time," the representative said.

Michael White flew from Los Angeles to Dubai on July 9, and then on to Iran, his mother said.

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He was visiting a woman whom he said was his girlfriend, she said. 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.

Michael White's wife, Lupe White, said she last saw her husband July 9, and he texted her a day later saying he was on a business trip for two weeks. Lupe White said he has had an online relationship with a woman in Iran for six years and would go see her for two weeks at a time.

She and her husband, who have been married for almost 15 years, have discussed divorce, she said, but she was hoping they had strengthened their relationship in the past six months as she helped him through chemotherapy.

"I still love him. He is my partner. He is still my husband," she said.

She said the State Department had not contacted her and she was unaware he was in prison until reached by CNN. White said she talked to police at her home. She called them when her husband did not return home, she said.

Other foreign nationals held in Iran

Michael White is one of several US citizens and other foreign nationals held in Iran. The State Department advises Americans not to travel to Iran "due to the risk of arbitrary arrest and detention."

Siamak Namazi, and his father, Baquer Namazi , are Iranian-Americans who have been detained since October 2015 and February 2016, respectively. The son was held while visiting relatives in Tehran; both were sentenced in 2016 to 10 years in prison for what the Iranian government said was "cooperating with Iran's enemies," which usually implies cooperating with the United States. American officials have demanded their release, saying the father and son did nothing wrong.

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US officials have also pushed Iran for information about the whereabouts of Robert Levinson , a former FBI agent and CIA contractor who went missing in Iran in 2007. Iran has denied having anything to do with his disappearance and said it doesn't know where he is. His family sued Iran over his disappearance in 2017

In December, six families of foreign or dual nationals held or believed to have disappeared in Iran -- including the families of the Namazis and Levinson -- issued an open letter alleging their relatives were political hostages detained without due process.

Recent releases

Iran has released Americans and other foreign nationals in recent years.

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In January 2016, Iran freed four US prisoners -- Washington Post journalist Jason Rezaian, Marine veteran Amir Hekmati, Christian pastor Saeed Abedini and Nosratollah Khosravi-Roodsari -- in exchange for the US granting clemency to seven Iranians indicted or imprisoned in the United States.

A fifth American, Matthew Trevithick, was separately released that month, US officials said.

Those releases came months after the international deal to restrict Iran's nuclear program -- the same deal from which the United States withdrew last year.