France demanded on Wednesday that Iran immediately release two of its nationals who have been held in prison since June, a situation that is likely to complicate Paris's efforts to defuse tensions between the United States and Tehran.
France's foreign ministry confirmed that Roland Marchal, a senior researcher from Science-Po university, was being detained.
French officials and his family had sought to keep the information secret due to the current disputes in the region, fearing it could harm potential negotiations.
Marchal's colleague, Franco-Iranian dual national Fariba Adelkhah, has been in prison in Iran since June.
"We want the Iranian authorities to show transparency in this dossier and act without delay to end the unacceptable situation," foreign ministry spokeswoman Agnes von der Muhll said in a briefing.
She added that Marchal had received consular visits and had a lawyer. Iran has refused to offer the same for Adelkhah, citing her Iranian nationality, and has called France's demands for her release an interference in its internal affairs.
Adelkhah and Marchal were arrested at a time when France and other European powers were caught up in an international standoff over Tehran's 2015 nuclear deal, which the United States abandoned last year.
Marchal, an Africa expert, had sought to spend the Eid religious holiday with his colleague in Iran, but was arrested at Tehran airport upon his arrival, their university association said in a statement.
"Nothing justifies the arrest of Fariba Adelkhah and Roland Marchal," the statement said. "Their activities are strictly academic and have no links whatsoever to any intelligence service and (they) do not carry out any political activity in Iran."
On Oct. 9 France issued a new advisory for Iran warning its nationals to postpone all travel plans, underscoring the risk posed by "the arbitrary arrest and detention practices of the Iranian security and intelligence services especially with regard to the contacts of foreign nationals with the population, notably for those in universities".
President Emmanuel Macron sought in September to broker a meeting between Iranian President Hassan Rouhani and U.S. President Donald Trump as he sought to defuse tensions between the two in recent months and convince Iran to comply with the nuclear deal. Those efforts faltered.
In another incident, Iran said on Monday agents of Revolutionary Guards had captured and returned to Iran a Paris-based journalist they suspect of fuelling anti-government street unrest across Iran last year using social media.
Spokeswoman von der Muhll on Wednesday also confirmed Rouhollah Zam had been given political asylum in France, but had no details on the circumstances surrounding his arrest outside of France.
"We recall our commitment to the respect for the rule of law, including freedom of expression and the right of asylum, and strongly condemn the arrest of Mr Zam," she said.