DUBAI (Reuters) - An American detained in Iran in July 2018 is being investigated on possible security charges, an Iranian prosecutor was quoted as saying on Friday, in a case that could further strain turbulent relations between Iran and the United States.

Ties between Tehran and Washington took a turn for the worse following U.S. President Donald Trump’s decision last May to pull out of an international nuclear deal and reimpose sanctions on Iran.

Several Americans have been detained in Iran in recent years, mostly on security-related charges including espionage. Trump warned in 2017 that Tehran would face “new and serious consequences” unless all unjustly held U.S. citizens were freed.

Gholamali Sadeqi, prosecutor in the northeastern city of Mashhad, said authorities were investigating security-related accusations against Michael White, according to the official news agency Mehr.

Sadeqi also said White, a 46-year-old U.S. Navy veteran, had been arrested because of a “private complaint” filed against him. In the past, “private complaints” filed by the Revolutionary Guards or other security bodies have led to arrests of activists and closure of newspapers.

Tehran has yet to respond to the release of an American-born anchor for Iran’s state-run Press TV who was detained in Washington. A U.S. court order on Thursday said she had been freed after testifying in an undisclosed federal investigation.

Iran confirmed White’s arrest earlier this month after the New York Times reported that White was arrested in July while visiting his Iranian girlfriend. London-based news website IranWire has reported that White was being poorly treated in prison, but Tehran has denied this.

White’s mother told the New York Times that her son, a California resident, suffers acute asthma and had undergone chemotherapy and radiation treatment for a neck tumor.