Facebook CEO Mark Zuckerberg. AP Photo/Pablo Martinez Monsivais Some of the web's biggest companies have unveiled new plans to crack down on extremist and terrorism-promoting material.

Facebook, Google-owned YouTube, Twitter, and Microsoft are stepping up efforts to remove extremist content from their websites by creating a common database.

The companies will share "hashes" — unique digital fingerprints they automatically assign to videos or photos — of extremist content they have removed from their websites to enable their peers to identify the same content on their platforms.

"We hope this collaboration will lead to greater efficiency as we continue to enforce our policies to help curb the pressing global issue of terrorist content online," the companies said in a statement on Tuesday.

Tech companies have long resisted outside intervention in how their sites should be policed, but have come under increasing pressure from Western governments to do more to remove extremist content following a wave of militant attacks.

The announcement comes after the European Commission issued the tech giants with a stark ultimatum on Sunday: Do more to tackle extremist content, or the EU will pass new laws forcing you to do so.

Many providers have relied until now mainly on users to flag content that violates terms of service. Flagged material is then individually reviewed by human editors who delete postings found to be in violation. Twitter suspended 235,000 accounts between February and August this year and has expanded the teams reviewing reports of extremist content.

Each company will decide what image and video hashes to add to the database and matching content will not be automatically removed, they said.

The database will be up and running in early 2017 and more companies could be brought into the partnership.

The European Union set up an EU Internet Forum last year bringing together the internet companies, interior ministers and the EU Counter-Terrorism Coordinator to find ways of removing extremist content.

The Forum will meet again on Thursday, when ministers are expected to ask the companies about their efforts and helping to provide evidence to convict foreign fighters.

"We are committed to protecting our users’ privacy and their ability to express themselves freely and safely on our platforms," the companies said in a joint statement shared on Facebook. "We also seek to engage with the wider community of interested stakeholders in a transparent, thoughtful and responsible way as we further our shared objective to prevent the spread of terrorist content online while respecting human rights."

Similar digital hashes are already used to identfy and block images of child abuse online. Used by Google, Twitter, and Facebook, it automatically flags up offending imagery and blocks it or prevents it being uploaded, without needing to be verified by a human moderator.