Major tech firms on Wednesday pledged to come up with new measures for stamping out violent extremist content on the internet, amid growing pressure from governments pushing for action in the wake of the massacres at two New Zealand mosques in March.

Executives from Google, Twitter, Facebook and Microsoft met with the leaders of 18 countries in Paris to unveil the "Christchurch Call" of voluntary commitments for online platforms.

It was named for the city where a gunman killed 51 people at two New Zealand mosques two months ago while broadcasting his rampage live on Facebook via a head-mounted camera.

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The social networking giant has faced withering criticism since the attack, after the horrific footage was uploaded and shared millions of times despite efforts to remove it.

"The call is a roadmap to action," New Zealand Premier Jacinda Ardern said at a press conference with French President Emmanuel Macron.

"It commits us all to build a more humane internet which cannot be misused by terrorists for their hateful purposes," she said.

READ MORE Jacinda Ardern to lead international summit tackling online extremism

What did the tech companies pledge?

Google, Microsoft, Facebook, Twitter and Amazon later released a nine-point plan for jointly putting the Christchurch pledges in motion, in particular for addressing the threat posed by livestreaming.

They promised investments in "digital fingerprinting" to track and remove harmful pictures and videos, and easy-to-use methods for users to report illicit content.

"This is leading to real action, and I think real action can play an important role in at least preventing a number of these kinds of attacks," Microsoft's president and chief legal officer Brad Smith told AFP in Paris.

"No one wants to see the internet used as the staging ground for these kinds of terrorist atrocities," he said.

Facebook had already promised Wednesday to tighten access to Facebook Live, in particular by denying the service to users who have shared extremist content.

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No US for now

The largely symbolic initiative is intended to keep up the pressure on social media companies, which face growing calls from politicians across the world to prevent their platforms from becoming stages for broadcasting extremist violence.

"Our goal is to never again see the internet transformed into a crazy propaganda machine, a goal sought by both terrorists on the far right and Islamic terrorists," Macron said.

A total of 18 governments signed the call: Australia, Canada, the European Commission, France, Germany, Indonesia, India, Ireland, Italy, Japan, Jordan, the Netherlands, New Zealand, Norway, Senegal, Spain, Sweden, and the UK.

But despite the endorsement from the biggest US tech firms, Washington will not join for now, even though "we continue to support the overall goals reflected," the White House said in a statement.

"We continue to be proactive in our efforts to counter terrorist content online while also continuing to respect freedom of expression and freedom of the press," it said.

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Macron appeared to take the refusal in stride, saying "The American administration has expressed its support for our effort, which I already consider progress."

The companies said they would cooperate on finding new tools to identify and quickly remove extremist content, such as sharing databases of violent posts or images to ensure they don't spread across multiple platforms.

They also said they would explore tweaking their algorithms to prevent violent or hateful content from going viral, while making it easier for users to report harmful posts.

But it will be up to companies to develop specific tools or policies.

'Can't prevent content'

Many countries have already tightened legislation to introduce penalties for companies that fail to take down offensive content once it is flagged, by either users or the authorities.

But analysts say the tighter controls pledged Wednesday will go only so far in preventing people from circumventing rules and policies already in place against disseminating violence and hate speech.

"You can't prevent content from being uploaded: it would require the resources for tracking everything put online by all internet users," said Marc Rees, editor in chief of the technology site Next INpact.

"Can you imagine trying to get TV or radio to prevent libellous, abusive or violent speech that someone might say?" he asked.

The "Christchurch Call" meeting ran in parallel to an initiative launched by Macron called "Tech for Good" which brought together 80 tech executives to discuss how to harness technologies for the common good.

But the US government was represented only at a junior level at a meeting of G7 digital ministers which also took place Wednesday in Paris.