France says a French-Iranian academic has been detained in Iran and denied consular assistance.
The French foreign ministry called on the Iranian authorities to "shed full light" on Fariba Adelkhah's situation and allow diplomats to visit her.
Ms Adelkhah, an anthropologist and researcher at Sciences Po university in Paris, was reportedly detained in June.
On Sunday an Iranian government spokesman told local reporters that he had "no information" about her case.
"I heard the news, [but] do not know who arrested her and on what grounds," Ali Rabiei was quoted as saying by the Tasnim news agency.
Iran has detained a number of dual citizens and foreign nationals in recent years in Iran, many of them on espionage charges. They include Nazanin Zaghari-Ratcliffe, a British-Iranian a project manager for the Thomson Reuters Foundation, and Xiyue Wang, a Chinese-American researcher at Princeton University.
Ms Adelkhah, who is 60, is a director of research at Sciences Po's Centre for International Studies (CERI). Her research focuses on social and political change in Iran during the second half of the 20th Century. She has written a number of books on Islam and Iran.
IranWire, an online news website run by Iranian expatriates, cited sources as saying Ms Adelkhah was arrested by Islamic Revolution Guard Corps (IRGC) intelligence agents in Tehran "on probable charges of espionage".
She had been carrying out research in Iran for several months and had spent time in the holy city of Qom along with a French student, IranWire reported.
When asked by reporters about Ms Adelkhah's case on Monday, a French foreign ministry spokeswoman confirmed that the ministry had been informed of her arrest.
The spokeswoman said France had "taken steps to get information from the Iranian authorities on her situation and the conditions of her arrest and asked for consular access", adding that "no satisfactory response has been given to these requests".
The Iranian authorities do not recognise dual nationality for Iranian citizens and do not grant consular access for foreign diplomats to visit them in detention.
The news about Ms Adelkhah comes as France and other European countries try to save a nuclear deal with Iran, which is close to collapse after the Islamic Republic breached several commitments in response to reinstated US sanctions.