Iran has released a jailed French researcher, in an apparent prisoner swap after France freed an Iranian threatened with extradition to the United States.

French President Emanuel Macron "is happy to announce the release of Roland Marchal, imprisoned in Iran since June 2019", his office said on Saturday. Marchal is due in France later in the day.

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France has for months demanded that Iran release Marchal and fellow researcher Fariba Adelkhah who were arrested last year on the charge of plotting against national security.

Meanwhile, the Iranian judiciary's news agency Mizan Online reported that Iranian Jallal Rohollahnejad had been freed by France on Friday. Iranian state television later said he was already on a flight back to Iran's capital, 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.

Iran has in recent months carried out prisoner exchanges with the US, Australia and Germany.

Coronavirus fears

In its statement, Macron's office also said the president "urges the Iranian authorities to immediately free" Adelkhah, a citizen of both Iran and France. Tehran does not recognise dual nationality.

Adelkhah, 60, 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.

"We welcome with relief the arrival of Roland Marchal in Paris after nearly nine months of arbitrary detention in very difficult conditions, but only half of the path has been taken," said Jean-Francois Bayart, a member of the committee and professor at the Geneva-based Graduate Institute of International and Development Studies.

He said "the fight continues" to secure Adelkhah's release. Adding to concerns for the welfare of the prisoners, Iran has been hit hard by the novel coronavirus pandemic, behind only Italy and China in the official number of deaths.

Iran said on Saturday that 123 more people had died from coronavirus, raising the country's official death toll to 1,556.

Prisoner releases

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 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 for charges including espionage and collaboration with the US government - his father Baquer and environmental expert Morad Tahbaz.

The Islamic republic in December freed Xiyue Wang, a US academic, in an exchange for scientist Massoud Soleimani and said it was open to further swaps.