BEIRUT - Shortly after the revolt against President Bashar al-Assad erupted in March 2011, Imad al-Souri quit his computer job to help the protests. He uploaded online videos of the marches and sneaked banned loudspeakers to demonstrators to amplify their voices calling for Assad's downfall.

Not anymore.

Souri, 28, recently fled to Turkey, fearing he would be killed or abducted by Islamic militants who are now the most powerful force in the rebellion and who are increasingly targeting those seen as opposed to their extremist ideologies. It's not an idle fear - dozens of activists have been abducted by radicals and, like Souri, dozens of those who shaped the initial uprising against Assad have fled.

"They want to liquidate me because I am a secular person," said Souri, speaking via Skype from his apartment on the Turkey-Syria border, which he shares with two other activists who also fled. "They are waiting for me to return to kill me." He spoke on condition he be identified by the nickname he uses as an activist for his own protection.

It's a depressing turn for antigovernment activists. At the start of the uprising, they worked in secrecy because of Assad's ruthless security services. Now they fear some of their once-presumed protectors: rebels who took up arms initially to defend protesters from the violent crackdown by Assad's forces.

The trend was highlighted by two reports issued Thursday.

In one, the rights group Amnesty International said one of the most powerful militant groups, the al-Qaeda-linked Islamic State in Iraq and the Levant (ISIL), is running secret prisons in territory it controls, carrying out torture and summary killings. Children as young as 8 are held along with adults in seven ISIL-run detention facilities in Aleppo province in the north and Raqqa province in the east, it said. Many detainees are held for challenging ISIL's rule, crimes like theft or for committing purported "crimes against Islam," such as smoking cigarettes.

Also, a U.N. panel investigating human-rights violations in Syria reported increasing hostage-taking operations by rebel groups, specifying ISIL - an act it described as a war crime. The panel also accused the Syrian government of possibly committing crimes against humanity - a more serious offense - for systematic disappearances of Syrians who are detained by government forces or pro-government militias and never heard from again.

Hard-line Islamic rebels are casting a shadow over parts of the country where they have wrested power. Abductions of moderate religious figures, humanitarian workers, human-rights defenders, journalists, and activists have increased since spring, according to more than a dozen activists and officials from human-rights organizations interviewed by the Associated Press.

Bassam al-Ahmad of the Violations Documentation Center, a Syrian group that tracks rights violations, estimated 40 activists were kidnapped, including his own colleagues. Another collective, the Aleppo Media Center estimated that in just November, at least 150 activists fled the country, three were killed, and 10 were abducted.

The numbers vary for several reasons. It's difficult to define who is a Syrian activist. Some activists are quickly released, making it difficult to keep track of the fluctuating numbers.

Souri was targeted after he joined a demonstration in Aleppo against a jihadi religious court. After a series of phone calls warning he would be killed, he was ordered by a mystery caller to leave Aleppo in 24 hours. Souri said he promptly fled on Oct. 28.

"The regime is shelling every day, and people are dying - but nobody knows of them, because activists fled," Souri said.

Amid growing infighting in rebel ranks, ISIL has also gone after activists affiliated with more moderate rebel factions to silence them.

For those who remain in Syria, it's a life of fear, said Fadi, an activist in Aleppo province. He spoke quietly about ISIL abuses from inside a hospital, asking that he be identified only by his first name for fear of retaliation. "If somebody hears me, if I speak - with ISIL, it's an execution," he said.

"We want to go back to the time when we were in a revolution" against Assad, he said.