France has for months demanded that Iran release Fariba Adelkhah and her partner, Roland Marchal, who were detained in June 2019 after being accused of plotting against national security.
President Emmanuel Macron announced on Saturday that Iran has freed a French researcher imprisoned in the Islamic republic after France released an Iranian threatened with extradition to the US.
Macron "is happy to announce the release of Roland Marchal, imprisoned in Iran since June 2019" but he "urges the Iranian authorities to immediately free" Fariba Adelkhah, an anthropologist, his office said.
France has for months demanded that Iran release Adelkhah and her partner Marchal, who were detained in June 2019, accused of plotting against national security. Their trial began in early March.
Adelkhah is a citizen of both Iran and France, but Tehran does not recognise dual nationality.
Iran has in recent months carried out prisoner exchanges with the US, Australia and Germany.
Marchal is due in France later Saturday, according to Macron's office.
Iranian judiciary's news agency Mizan Online reported Iranian Jallal Rohollahnejad had been freed by France on Friday. Iranian state TV later said he was already on a flight back to Tehran.
Rohollahnejad, "an Iranian engineer incarcerated for more than a year in French prisons and accused of circumventing American sanctions against Iran, has been freed today," the news agency added.
The French Court of Cassation had on March 11 approved "the request to extradite Rohollahnejad to the US, but the French government freed him, changing this decision", it added.
"Taking into account the cooperation of the (Iranian) judicial system's intention to release a French detainee through reducing sentences, the French government" freed the Iranian engineer "in an act of mutual cooperation", according to the report.
Adelkhah, an anthropologist and expert on Shia Islam, faces charges of "propaganda against the system" and "colluding to commit acts against national security", according to the researchers' lawyer, Said Dehghan.
Her colleague Marchal, a specialist on East Africa, is accused of the same national security charge, said the lawyer.
Their Paris-based support group and the French foreign ministry had sounded the alarm over the health of both detainees – Adelkhah went on hunger strike for 49 days and Marchal's health is said to be deteriorating.
The support group has repeatedly said that the two are innocent of the charges they face.
Adding to concerns for the welfare of the prisoners, Iran has been hard hit by the novel coronavirus pandemic, behind only Italy and China in the official number of fatalities.
The death toll in the Islamic republic rose by 149 to 1,433, according to the latest official figures released on Friday.
Ahead of Iran's celebration of the Persian New Year starting Friday, authorities had released a number of international prisoners.
US Navy veteran Michael White was freed on Thursday.
He was handed over in the northeastern Iranian city of Mashhad to a team from Switzerland, which represents US interests in the absence of diplomatic relations, and flown to the capital Tehran, the US State Department said.
Iran this week also freed for two weeks Nazanin Zaghari-Ratcliffe, a British-Iranian dual national who worked for the Thomson Reuters Foundation, the media organisation's philanthropic arm.
Iran is still holding US citizens Siamak Namazi – who was convicted on charges that include espionage and collaboration with the US government – his father, Baquer, and environmental expert Morad Tahbaz.
Tehran in December freed Xiyue Wang, a US academic, in an exchange for scientist Massoud Soleimani and said it was open to further swaps.