France's foreign ministry on Friday said it had summoned the Iranian ambassador over the "intolerable" detention of two French academics, expressing "grave concern" that one was now on hunger strike.
The envoy "was reminded of France's demand that Fariba Adelkhah and Roland Marchal are released without delay and that the Iranian authorities show total transparency over their situation," it said in a statement.
"As the President (Emmanuel Macron) and Foreign Minister (Jean-Yves Le Drian) have said on several occasions, their imprisonment is intolerable," it added.
Adelkhah and Marchal, both academics at Sciences Po in Paris, have been held by the Iranian authorities since June. The university said this month that Adelkhah and another detained academic, Australian Kylie Moore-Gilbert, had started a hunger strike.
The French statement said the ministry had made clear to the ambassador "our grave concern over the situation of Mrs Fariba Adelkhah, who has stopped taking food".
It also said the ministry reaffirmed France's demand of allowing consular access, a request which has so far been refused. Iran does not recognise dual nationality.
"The French authorities will continue to act with determination to obtain their release," it added.
A specialist in Shia Islam and a research director at the university, Adelkhah was arrested on charges of "espionage" that have been rubbished by her supporters.
Marchal had come to Iran to visit Adelkhah and stands accused of "collusion against national security", according to his lawyer.
Iranian media said earlier this month their bid to be released on bail was rejected and their case will now go before the Revolutionary Court which handles high-profile cases in Iran.
Their imprisonment has added to tensions between Tehran and Paris at a time when Macron is seeking to play a leading role in defusing tensions in the standoff on the Iranian nuclear programme.
Melbourne University academic Moore-Gilbert's detention on charges of "spying for another country" was confirmed in September, but her family said at the time that she had been detained for months before that.
In an open letter, Moore-Gilbert and Adelkhah, who are sharing a cell said they were subjected to "psychological torture" and called for international solidarity in the name of "academic freedom".
The latest tensions come after Xiyue Wang, an American scholar who had been serving 10 years on espionage charges, was released by Iran earlier this month in exchange for Massoud Soleimani, an Iranian who had been held in the US for allegedly breaching sanctions.
Iran has said it is open to more such prisoner swaps with the United States.
Tehran is still holding several other foreign nationals in high profile cases, including British-Iranian mother Nazanin Zaghari-Ratcliffe and Iranian-American businessman Siamak Namazi and his father Mohammad Bagher Namazi.