A senior Syrian rebel commander suspected in the disappearance of a number of prominent human rights activists has been arrested in France.

Islam Alloush - a former spokesperson for Jaish al-Islam, a group largely based in the Damascus suburb of eastern Ghouta - was detained in the city of Marseille, a judicial source told AFP on Friday.

The arrest follows the filing of a criminal complaint last year by human rights groups against members of Jaish al-Islam, accusing them of war crimes including torture and kidnapping.

Jaish al-Islam has long been suspected of involvement in the disappearance of Syrian human rights activist Razan Zaitouneh, her husband Wael Hamada and two colleagues, Samira Khalil and Nazem Hammadi.

Following three years of research, the human rights groups were able to signal to the French authorities in January the presence of Alloush - whose real name is said to be Majdi Mustafa Nameh - in the south of France.

The human rights organisation FIDH, one of those which filed the original complaint, said that they hoped the arrest of Alloush could help uncover the activists' fates.

"Razan’s peaceful commitment, righteousness and values remain emblems of the hopes raised by the early Syrian democratic uprising," said Clémence Bectarte, Lawyer and Coordinator of FIDH’s Litigation Action Group.

"The Syrian people deserve to know what finally happened to her."

Unknown fates

Following the uprising against Syrian President Bashar al-Assad in 2011, Zaitouneh became one of the most prominent civil society figures campaigning against his government's crackdown on the opposition.

She was also a fierce critic of abuses carried out by rebel groups, which some suspect earned her the ire of Jaish al-Islam.

The group, originally seen as close to Saudi Arabia and funded at times by Turkey, advocated an ultra-conservative brand of Islamism, and leader Zahran Alloush was known for making sectarian comments against Alawites and Shia.

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The group were, however, fierce opponents of the Islamic State group, with whom they clashed on a number of occasions.

In a statement with other human rights groups, FIDH said Alloush was allegedly involved in "the forced enlistment of children in the armed group" and said "several victims also directly incriminate him for kidnapping and torture".

They added that Jaish al-Islam carried out a "reign of terror" in the areas it controlled and numbered more than 20,000 fighters.

The group lost control of eastern Ghouta in April 2018 following an assault by the Assad government.

Writing for Middle East Eye last year, Samira Khalil's husband Yassin al-Haj Saleh said that he still hoped to find out what happened to his wife and called on Jaish al-Islam's backers and international monitors to intervene.

"Samira’s absence is a big loss, but she is my motivation to go on, and to go deep, in this desperate struggle for freedom, human dignity and justice," he wrote.

"At the final hour of 2019, it is time for the United Nations and Turkey to take concrete steps to uncover the fate of my wife, our friends and the tens of thousands of others who have gone missing."