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A French academic held in Iran for four months now faces charges of collusion after he and his French-Iranian colleague were arrested, their lawyer said in a report on Monday.

Roland Marchal, a sociologist at Sciences Po in Paris, was arrested in June together with Fariba Adelkhah, also an academic at the prestigious university, and who Iran accuses of being a spy.

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.

Read more: Iran's 'farcical' spy series retells story of detained WaPo journalist

However, Dehghan added that the reasons for the charges were still unknown.

"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 cases of the two respected academics 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 levelled allegations," said Dehghan.

The French government has condemned the arrest of Marchal, a sociology specialist 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 Shia 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 recognise dual nationality, has criticised 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.