Iran has been holding an American Navy veteran in prison on unspecified charges since late July, when he was seized while visiting an Iranian girlfriend, his mother said Monday.

The imprisonment of the veteran, Michael R. White, 46, from Imperial Beach, Calif., could further complicate relations between the United States and Iran. Tension between the countries worsened substantially after President Trump renounced the nuclear accord with Iran last May and reimposed severe sanctions.

At least three other American citizens, two of them of Iranian descent, have been incarcerated in Iran for years. Another American has been missing in Iran for more than a decade.

Image Michael R. White, an American Navy veteran, in an undated photograph. Credit... via Joanne White

Mr. White’s mother, Joanne White, said in a telephone interview that he had been set to return from Iran via Dubai, in the United Arab Emirates, on July 27, but never boarded his flight.