Mothers of the detainees gather every day outside ISIL bases in the city of Raqa, where the extremist group is in full control and governs through a harsh interpretation of Islamic sharia law, according to residents and activists.
"They cry, begging for information and for their sons' release," said Amer Matar, whose citizen journalist brother Mohammad Nour has been detained by ISIL for nine months.
"My mother suffers every day, because she is not given any information about her youngest child," said Matar, a filmmaker from Raqa who became a refugee in Germany because of his own activism against President Bashar al-Assad.
Rooted in Al-Qaeda in Iraq, ISIL first emerged in Syria's war in spring last year, and was initially seen as an ally by fighters seeking Assad's overthrow.
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But ISIL's quest for hegemony and systematic abuses - including the kidnapping of hundreds of rebels, activists and even ordinary citizens accused of "crimes" like heresy and smoking cigarettes - eventually turned the opposition against the extremist group.
ISIL has in recent months been expelled by rival rebels from many opposition areas, but the northeastern city of Raqa on the Euphrates river remains squarely under its control.
She also said those suspected of opposing ISIL or violating its puritanical social code vanish, all too often without a trace, while others have been publicly executed.
The province is home to an unknown number of detention facilities, including secret prisons where torture is especially severe, says Nassar, who works with the Syrian Network for Human Rights.
Activists describe a climate of fear not unlike that under the Assad regime, and many have fled to nearby Turkey, fearing arrest or worse.
Despite the dangers, a group of dissidents using secret identities last week launched a campaign calling on ISIL to leave Raqa under the name 'Raqa is Being Slaughtered Silently'.