Iran's Foreign Ministry confirmed on Wednesday that an American citizen had been arrested in the city of Mashhad, but denied reports he had been badly treated, according to the Islamic Republic News Agency.

Foreign Ministry Spokesman Bahram Ghasemi was quoted by the news agency as saying that Michael White, a 46-year-old US Navy veteran, was arrested "some time ago" in Iran's second largest city.

Ghasemi said the United States government, through the Swiss embassy in Tehran, was informed of the arrest during the early days of the incident.

US interests in Iran are handled by the Swiss diplomatic mission, as the two countries have had no diplomatic relations since 1980.

White, 46, from Imperial Beach, California, was arrested in July after visiting his girlfriend in Mashhad, his mother told The New York Times earlier this week.

The US State Department said on Tuesday it was "aware of reports" of his detention, but declined to release details, citing privacy considerations.

Ghasemi said reports in the US media that White was being held in poor conditions were "false and inaccurate".

"The case of this person is being handled by the relevant authorities and at the conclusion of this process they will issue the necessary information," Ghasemi was further quoted as saying on his Telegram account.

White joins at least three other US citizens, two of them of Iranian descent, being held in Iran.

In July 2017, Iran said that it had arrested Chinese-American, Xiyue Wang, a Princeton graduate student who was conducting research in Iran.

Since October 2015, it has also been holding Iranian-American Siamak Namazi. Namazi's father, Baquer, an 82-year-old former diplomat for the United Nations Children's Fund (UNICEF), is also being held. Both are naturalised American citizens.

In 2007, a former FBI agent, Robert Levinson, went missing in Iran.

The prisoner issue has long added to the tensions between Iran and the United States.

The last time Iran released any imprisoned Americans was when the international accord on Iran's nuclear programme took effect in January 2016.

The Iranians released four Americans, including Jason Rezaian, the former correspondent for The Washington Post. In exchange, the US released several Iranians held on sanctions violations.

Relations between Iran and the US deteriorated sharply last year when President Donald Trump withdrew from the nuclear deal and re-imposed sweeping sanctions on Iran.