Major tech firms have pledged to pursue a range of new measures aimed at stamping out violent extremist content on the internet, amid at growing pressure from governments in the wake of the massacres at two New Zealand mosques.

The "Christchurch Call" was spearheaded by New Zealand's premier Jacinda Ardern and French leader Emmanuel Macron, who gathered tech executives and world leaders to launch the initiative at a meeting in Paris.

The vow came after a gunman killed 51 people at two mosques in Christchurch on 15 March while broadcasting his rampage live on Facebook via a head-mounted camera.

"The dissemination of such content online has adverse impacts on the human rights of the victims, on our collective security and on people all over the world," the signatories said in a statement.

Facebook in particular has faced withering criticism since the Christchurch attack, after the horrific footage was uploaded and shared millions of times despite efforts to remove it.

The social media giant, which participated in crafting the new commitments, said earlier that it would tighten access to its livestreaming feature.

Google and its YouTube unit also joined the pledge, along with Twitter, Wikipedia, Dailymotion and Microsoft.

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 do not 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.

"For the first time, governments, international organisations, companies and digital agencies have agreed on a series of measures and a long-term collaboration to make the internet safer," Mr Macron's office said in a statement.

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

51 people were killed in the attacks in Christchurch last March

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.

"We need to get in front of this (problem) before harm is done," Ms Ardern told CNN.

"This is not just about regulation, but bringing companies to the table and saying they have a role too."

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.

Facebook said it would ban Facebook Live users who shared extremist content and reinforce its internal controls to stop the spread of offensive videos.

"Following the horrific recent terrorist attacks in New Zealand, we've been reviewing what more we can do to limit our services from being used to cause harm or spread hate," Guy Rosen, the firm's vice-president for integrity, said in a statement.

Meanwhile, the United States has said that it will not join the international bid to stamp out violent extremism online.

However, the White House stressed that Washington backs the initiative's aims.

"While the United States is not currently in a position to join the endorsement, we continue to support the overall goals reflected" in the so-called "Christchurch Call," the White House said.

Varadkar addresses Paris summit on online extremism

Taoiseach Leo Varadkar has told the Paris summit that Ireland's experience with extremist violence is a reminder of what he called the "devastating impact" of radicalisation on families, communities and society.

Mr Varadkar told the 'Christchurch Call' meeting that the attack on Muslims in the New Zealand in March was an attack on "our collective international values, our respect for different traditions, and our shared humanity."

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This morning, Facebook announced it is tightening access to live-streaming to prevent the rampant sharing of graphic video as took place with the Christchurch mosque attacks.

A 'one-strike' policy at Facebook Live will be applied to a broader range of offences, with those who violate serious policies suspended from using the feature after a single breach.

Facebook says technical innovation is needed to get ahead of the kind of media manipulation seen after the New Zealand attacks, such as users modifying videos in order to bypass filters.

On Friday, Facebook's Mark Zuckerberg said the internet should now face more public regulation.

Additional reporting Tony Connelly