BEIRUT -- Syria's civil war has been one of the modern world's most brutal conflicts and one of its most heavily filmed. Hundreds of thousands of amateur videos uploaded to YouTube document every heartbeat of the war over the past seven years, from momentous events like cities under bombardment to intimate scenes like a father cradling his dead children.

Syrian activists fear all that history could be erased as YouTube moves to rein in violent content. In the past few months, the online video giant has implemented new policies to remove material considered graphic or supporting terrorism, and hundreds of thousands of videos from the conflict suddenly disappeared without notice. Activists say crucial evidence of human rights violations risks being lost -- as well as an outlet to the world that is crucial for them.

Activists are rushing to set up alternative archives, but they also recognize nothing can replace YouTube because of its technological infrastructure and global reach.

"It is like we are writing our memories -- not in our own book but in a third party's book. We don't have control of it," said Hadi al-Khatib, co-founder of the Syrian Archive , a group founded in 2014 to preserve open source evidence of crimes committed by all sides of the Syrian conflict.

Get Breaking News Delivered to Your Inbox

Based on his database and review of around 900 groups and individuals, al-Khatib said some 180 channels connected to Syria were shut since June, when YouTube began using machine learning protocols to sift through videos on the site for objectionable content.

Working with YouTube, al-Khatib's group secured the return of about 20 channels, salvaging about 400,000 videos. But about 150,000 videos remain in jeopardy, pending a decision from YouTube, which is still reviewing whether to reinstate them, he said.

"Nothing is lost forever yet," al-Khatib said, speaking from Berlin. "But this is very dangerous, because there is no alternative for YouTube."

YouTube, which is owned by Google, says it will correct any videos improperly taken down and that it is in dialogue with the activists on a solution. But many activists fear a repeat or a permanent loss. The shutdowns were chilling for a community that had just celebrated a possible precedent for Syria when the International Criminal Court in August issued an arrest warrant based on video evidence for a Libyan military commander.

One prominent Syrian human rights group, the Video and Documentation Center in Syria, said it will stop using YouTube and will set up its own storage and platform. "The risk became very big now and we don't trust this platform anymore for keeping violations evidence," Husam AlKatlaby, VDC executive director, said in an email.

VDC, registered in Switzerland, has specialized in documenting rights violations since 2011. The group limited access to its YouTube channel since 2014, making it a closed channel, after the company warned it over graphic content.

But not everyone can afford to go on their own. Also, YouTube provides activists with personal accounts for free and technological tools to edit, translate and upload anytime - vital for people taking video out in the field in dangerous circumstances.

Activists used YouTube first to report on the peaceful protests that erupted in 2011 against the rule of President Bashar Assad, using videos taken on mobile phones. As the conflict got bloody, so did the videos, catching the aftermath of chemical attacks, spectacular aerial bombings, rescuers pulling children from rubble, and new strikes hitting rescuers and survivors. Militant groups uploaded videos of beheadings. Government supporters uploaded their own imagery and propaganda.

Often, the images were the only thing to grab the world's attention in an intractable conflict. A video last year that was viewed more than 4.3 million times showed a child covered in blood and dust after surviving an airstrike in Aleppo, as government forces advanced to recapture the city.

YouTube previously relied in part on a system of community flagging of content deemed inappropriate.