Roland Marchal, a French academic held in Iran for four months, faces charges of collusion after being arrested alongside Fariba Adelkhah, a Franco-Iranian colleague accused of espionage, their lawyer said in a report Monday.
Marchal, a researcher at Sciences Po in Paris, was arrested in June together with Adelkhah, an academic at the same university.
Marchal had come to Iran to visit Adelkhah and was accused of "collusion against national security", Saeed Dehghan said, quoted by the semi-official ISNA news agency.
But the lawyer said the reasons for the charges were still unknown to him.
"Up until this moment, the reasons for my clients' charges, which according to Article 32 of the Constitution must be given alongside the charges, have not been mentioned," Dehghan said.
"It seems that Mr Marchal has been arrested because of his friendship with Ms Adelkhah," he added.
The lawyer said the two cases were still at the level of prosecutors and had not yet been sent to court.
"We are still in negotiations with judicial authorities and we hope misunderstandings in this regard will be removed, as up until now no reason has been given for the leveled allegations," said Dehghan.
The French government has condemned the arrest of Marchal, a sociologist whose research focuses on civil wars in Africa.
"We urge the Iranian authorities to be transparent and act without delay to put an end to this unacceptable situation," the foreign ministry said on October 16.
The arrest of Adelkhah, a specialist in Shiite Islam, was confirmed on July 16 by Iran's judicial spokesman Gholamhossein Esmaili, who gave no further details.
Her friends and colleagues in France have said she was accused of espionage.
Tehran, which does not recognize dual nationality, has criticized Paris for "unacceptable interference" in its domestic affairs after the French government sought consular access to Adelkhah.
The pair are not the only academics in Iranian custody.
On October 1, Iran confirmed the arrest of anthropologist Kameel Ahmady, reportedly a British-Iranian dual national.