The White House will not sign an international agreement to combat online extremism brokered between French and New Zealand officials and top social media companies, amid US concerns that it clashes with constitutional protections for free speech.

The decision comes as world leaders prepare to announce the so-called "Christchurch call to action" on Wednesday, an effort named after the city where a shooter attacked two mosques in an attack inspired by online hate and broadcast on social-media sites.

The document calls on governments and tech giants to improve their efforts to study and stop the spread of harmful content.

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US officials said they stand "with the international community in condemning terrorist and violent extremist content online," and support the goals of the Christchurch document.

But the White House said in a statement it is "not currently in a position to join the endorsement," which leaders from countries such as Australia, Canada and the United Kingdom are expected to sign.

The decision puts the United States at odds with US tech companies including Facebook and Google, which are expected to support the effort.

A day earlier, White House officials raised concerns that the document might run afoul of the First Amendment.

"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," the White House said.

"Further, we maintain that the best tool to defeat terrorist speech is productive speech, and thus we emphasise the importance of promoting credible, alternative narratives as the primary means by which we can defeat terrorist messaging."

New Zealand Prime Minister Jacinda Ardern and French President Emmanuel Macron organised the call to action, to be unveiled later Wednesday in Paris, as a response to Ardern's plea for greater social media accountability after a shooter in March streamed the Christchurch attack live for millions to see online.

Facebook, Google and Twitter struggled to swiftly take down copies of the violent video as fast as it spread on the web, prompting an international backlash from regulators who felt malicious people had evaded Silicon Valley's censors too easily.

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Entering the summit, Facebook announced two efforts to address regulators' concerns and stop the spread of harmful content on its services.

Now, users who violate Facebook's "most serious policies" - such as sharing a link to statements from a known terrorist group - would be banned from broadcasting live videos on the platform for set periods of time.

Facebook said the policy, if implemented sooner, might have stopped the Christchurch shooter from using the company's live-streaming feature to stream the attacks on the mosques.