DUBAI, United Arab Emirates (AP) - A British-Iranian anthropologist who has studied child marriage and female genital mutilation in the Islamic Republic has been detained in Iran over unknown charges, his wife and activists said Wednesday, becoming the latest dual national held there amid tensions with the West.

Kameel Ahmady is imprisoned in Tehran’s Evin prison, where other dual nationals detained by Iran’s paramilitary Revolutionary Guard are held, and is the first British citizen known to be held since Prime Minister Boris Johnson took office last month.

Although his detention has been unacknowledged by Iranian authorities, it fits a wider pattern described by families of those detained and United Nations investigators of Tehran holding those with Western ties as chips for future negotiations.

Ahmady was been held since Sunday, his wife Shafagh Rahmani said on Instagram. She said Ahmady faced a one-month detention order as unknown investigators pursue a case against him.

“There is no given information about his charges or why he was arrested,” she wrote.

The New York-based Center for Human Rights in Iran also spoke to Ahmady’s wife and confirmed his detention. It said Ahmady previously had been the target of hard-liners in the media for his work on “on politically sensitive topics including child marriage, LGBTQ issues and female genital mutilation.”

“His research directly contradicts the views and statements of powerful clerics and political and religious conservatives who made and back policies that can expose vulnerable members of society to abuse and exploitation,” the center said.

Iranian state media have not acknowledged Ahmady’s detention, which is common in the days immediately after such an arrest. Iran’s mission to the United Nations and the British Foreign Office did not respond to requests for comment Wednesday.

Ahmady is just the latest dual national detained amid heightened tensions between Iran and the West over its nuclear program. President Donald Trump unilaterally withdrew the U.S. from the accord last year. Iran recently has begun inching away from the accord, warning it will take further steps if Europe cannot guarantee it the ability to sell its crude oil on the global market.

Johnson already has faced controversy in dealing with Iran detaining dual nationals. He complicated efforts to free Nazanin Zaghari-Ratcliffe by saying incorrectly while serving as foreign minister that she was training journalists when arrested. He later apologized.

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