PARIS (AFP) - France said on Friday (Jan 10) that the imprisonment of two prominent French academics by Iran was unacceptable and that their release would represent a "significant gesture", as tensions mount between Teheran and the West.

The plea for the release of Fariba Adelkhah and Roland Marchal - imprisoned since June last year - comes as Iran is embroiled in an international crisis over its missile attacks on US troops in Iraq and a passenger plane crash near Teheran.

"These arrests and the fact that they are in prison today is totally unacceptable," French Foreign Minister Jean-Yves Le Drian told RTL radio. "It would be a significant gesture if Iran freed them as soon as possible."

France has repeatedly called for de-escalation in the latest intensification of the Iran-US standoff, sparked by the killing of top Iranian commander Qassem Soleimani in an American drone strike which prompted Iran to attack bases in Iraq housing US troops.

A Ukrainian passenger plane came down shortly after takeoff on Wednesday, with Western officials suggesting it had been downed - possibly accidentally - by an Iranian missile.

France, Britain and Germany are scrambling to keep alive a 2015 deal that reined in Teheran's nuclear programme, which US President Donald Trump walked out of in 2018.

Iran has dropped espionage charges against Adelkhah but she still faces charges of spreading "propaganda against the political system" and "conspiracy against national security", her lawyer said this week.

Iran does not recognise Adelkhah's dual French-Iranian nationality and has lashed out at Paris for what it has described as "interference" in the cases of the academics, both from Sciences Po university in Paris.

Marchal is also accused of "collusion against national security", according to his lawyer.

The two researchers are not the only foreign academics behind bars in Iran - Australian Kylie Moore-Gilbert of the University of Melbourne is serving a 10-year sentence on espionage charges.

Sciences Po said late last year that Adelkhah and Moore-Gilbert had begun a hunger strike. Moore-Gilbert has issued a plea to Prime Minister Scott Morrison to work for her release.

Teheran is still holding several other foreigners in high-profile cases, including British-Iranian mother Nazanin Zaghari-Ratcliffe and Iranian-American businessman Siamak Namazi and his father, Mohammad Bagher Namazi.