Facing mounting pressure to use their technological clout to curb the spread of jihadist propaganda, major US social networks have finally joined forces in an effort to curb "terrorist content."

Facebook, Twitter, Microsoft and Google-owned YouTube announced Monday a drive to stop the proliferation of videos and messages showing beheadings, executions and other gruesome content, posted by the likes of the Islamic State group or Al-Qaeda.

The move comes as social media giants face increasing scrutiny over their role in the explosion of so-called "fake news" -- which is believed to have influenced the US election -- as well as online bullying and hate speech.

Platforms such as Facebook, Twitter and YouTube have struggled for years with this issue, seeking to be open to free speech without being used to promote violence or hate.

But with the rampant use of the networks by jihadists to plan, recruit and depict violent attacks, the tech platforms were forced to take a stronger stand.

"There is no place for content that promotes terrorism on our hosted consumer services," they said in a joint statement.

James Lewis, a senior fellow who follows technology and security issues at the Center for Strategic and International Studies, believes social media have reached a turning point, and can no longer claim to be "neutral platforms."

"They can't evade responsibility anymore," Lewis said.

"Terrorist content is only the start," he said. "Now they have to figure what to do about hate speech, racism and bullying."

Monday's joint statement did not indicate what type of technology would be used in the new initiative, except to say it would be based on a shared industry database of "hashes" or digital fingerprints that identify jihadist content.

Some critics have suggested that such content could be curbed by using a template could be a program that is already employed by online firms to block child pornography.

- No easy answer -

Peter Weinberger, a senior researcher at the National Consortium for the Study of Terrorism and Responses to Terrorism at the University of Maryland, said there is no simple technological solution to the problem of jihadist content.

"It's a little more difficult to do than child pornography where you have very clear, objectionable content," he said.

"A lot of times terrorist recruiters can package stuff in a more savvy way. They use speech which is coded."

Weinberger said the effort "is commendable but they will need to have designated personnel who will need to constantly review material because it is always changing."

Earlier this year, Twitter announced it had suspended 360,000 accounts, mostly linked to the Islamic State group, as part of a stepped-up effort.

But research has shown that jihadist groups quickly migrate to new accounts, taking advantage of the anonymity of Twitter.

Lewis said it would appear feasible to implement the new initiative because the companies could rely on international treaties which define terrorism and organizations supporting those efforts.

Governments in the United States, France and elsewhere have been pressing online firms to do more to curb extremist content, especially videos that depict beheadings and other gruesome acts often used in recruitment efforts.

At least one lawsuit filed in France on behalf of a victim in the 2015 Paris attacks seeks to hold Facebook, Google and Twitter liable for the violence.

- Civil liberties issue? -

But Chris Calabrese of the Center for Democracy & Technology, an online civil liberties advocate, called the new effort "worrisome."

"We think the precedent of governments leaning on companies to do any centralized censorship could lead to a lot of negative consequences," Calabrese said.

"It's not just extreme and barbaric content that governments want to suppress. It's lots of other types of content like hate speech and copyright violations. It's a slippery slope."

Calabrese said that "there is no guarantee the program will work" in curbing the spread of violence and extremist content.

He said that to ensure the program is not abused, "companies should not take any censorship requests from governments," and there should be an appeal mechanism "for correcting any mistakes."