A senior member of Syrian Islamist armed group Jaysh al-Islam implicated in the disappearance of a prominent activist in 2013 has been arrested in France and charged with war crimes and torture, a judicial source told AFP on Friday.

The suspect, a former spokesman of the group who is in his early thirties, was in France on an Erasmus student visa and detained in the southern city of Marseille.

He appeared before an investigating magistrate in Paris who charged him with torture, war crimes and complicity in forced disappearances, said the source.

Jaysh al-Islam is one of several hardline Islamist groups opposing the Damascus regime that have emerged during the Syrian civil war.

It has fought against Islamic State (IS) jihadists but also been accused by rights groups of abuses.

It is notably suspected of involvement in the December 2013 kidnapping of the Syrian activist Razan Zaitouneh, her husband Wael Hamada and two colleagues, Samira Khalil and Nazem Hammadi. They have never been found.

Zaitouneh was one of the most prominent civil society figures in the uprising against President Bashar al-Assad that erupted in March 2011 as part of the Arab spring but then descended into civil war.

That year, she was awarded the prestigious Sakharov prize for human rights along with other activists. She was a vociferous critic of abuses by all parties in the conflict including the regime and groups like Jaysh al-Islam.

- 'New chapter in quest' -

Rights groups including the International Federation for Human Rights (FIDH) in June last year filed a criminal complaint against members of Jaysh al-Islam for crimes committed by the group.

After three years of painstaking research, they were able to signal to the French authorities in January the presence of the former spokesperson of the group in the south of France.

The FIDH and two other groups behind the complaint said in a statement that the man arrested is called Majdi Mustafa Nameh, although he operates under the nom-de-guerre of Islam Alloush.

They said Islam Alloush is a former captain in the Syrian armed forces, who then defected and became a senior official and spokesman for Jaysh al-Islam, close to its leader Zahran Alloush.

"Islam Alloush was also allegedly involved in the forced enlistment of children in the armed group. Several victims also directly incriminate him for kidnapping and torture," their statement said.

The move comes after the French authorities in 2018 issued arrest warrants against three high-ranking members of the Assad regime.

"The indictment of one of the former top officials of Jaysh al-Islam... opens a new chapter in the quest to prosecute international crimes committed in Syria since 2011," said Michel Tubiana, lawyer and Honorary President of Human Rights League (LDH), another of the groups which filed the complaint.

According to the statement by the three groups, Jaysh al-Islam numbered up to more than 20,000 fighters and carried out a "reign of terror" in the rebel areas it controlled, mainly in the Eastern Ghouta, until it lost control over them in April 2018.