France's Foreign Ministry confirmed on Wednesday that French researcher Roland Marchal is being held by Iranian authorities, saying it had asked Iran to end the "unacceptable" situation without delay.
Earlier, Marchal’s colleagues said he was arrested in June when he traveled to Iran to visit his partner, Fariba Adelkhah.
Marchal, a sociologist whose research focuses on civil wars in Africa, and Adelkhah, an anthropologist, both work at the Sciences Po university in Paris.
Iranian authorities disclosed in July that they had arrested Adelkhah, who holds dual French-Iranian nationality, on charges that have not been made public.
It's unclear exactly what charges Marchal faces, but Sciences Po professor Richard Banegas told The Associated Press that he and his colleagues consider him "an academic prisoner."
The French Foreign Ministry demanded Wednesday that Iran refrain from entering a new phase of "especially worrying" reductions to Tehran's obligations to a 2015 nuclear deal with world powers.
"Iran must abstain from crossing an especially worrying new phase of new measures that could contribute to an escalation in tensions," Ministry spokeswoman Agnès von der Muhll told reporters in a daily briefing.
She was responding after Iranian President Hassan Rouhani said on Monday that Tehran was working on advanced IR-9 centrifuges for uranium enrichment. Those centrifuges do not appear in the 2015 accord.
The French Ministry also condemned the arrest of Iranian activist Rouhollah Zam, saying he had refugee status in France.
It said it had no details of the circumstances surrounding Zam's arrest.
The Iranian Revolutionary Guards claimed on Monday that Zam, who ran a "counter-revolutionary" Telegram channel, was detained in a "sophisticated and professional operation" by the Guards' intelligence organization.
Zam reportedly lived in exile in Paris, but the Guards' statement did not specify when or where he was arrested.