Founded by Prime Minister Jacinda Ardern and French President Emmanuel Macron in the wake of the 2019 Christchurch mosques terrorist attack, the US previously declined to join the international agreement, citing freedom of speech concerns.

In a statement issued on Saturday morning (AEST), White House press secretary Jen Psaki said it retained some concerns while joining up.

"Countering the use of the internet by terrorists and violent extremists to radicalize and recruit is a significant priority for the United States," she said.

"In joining the Christchurch Call, the United States will not take steps that would violate the freedoms of speech and association protected by the First Amendment to the U.S. Constitution."

Signatories to the call will meet virtually next week in the second anniversary summit.

The Call, which counts Australia, the EU, Japan and India among its foundation supporters, is Ms Ardern's flagship foreign policy.

Facebook, Google, Twitter and Microsoft have also signed on.

The Call aims to bring together countries and global tech giants to stop the spread of extremist content online, aiming to prevent similar attacks as the 2019 Christchurch Mosques attack.

The man responsible for that massacre, Australian-born terrorist Brenton Tarrant, was radicalised online and livestreamed his crimes on social media.

Ms Ardern welcomed the United States' reversal.

"This announcement is a formalisation of that relationship and a commitment for us to work even more closely on eliminating terrorist and violent extremist content online," she said.

Australian Associated Press