The White House said on May 15 it supported the Christchurch Call's aims but was "not in a position to join", citing the need for freedom of speech.
The comments came as five of the world's biggest tech companies pledged to tackle extremist material.
The Christchurch Call was launched in response to a deadly terror attack that was live streamed on Facebook.
The March attack launched by a lone gunman on two mosques in the New Zealand city of Christchurch left 51 people dead.
The Christchurch Call is a voluntary commitment by governments and tech companies to improve their efforts to tackle extremist content.
It was spearheaded by New Zealand Prime Minister Jacinda Ardern and French President Emmanuel Macron, who joined political and industry leaders in Paris on Wednesday to launch the action.
The text of the initiative outlines "collective, voluntary commitments" from governments and internet companies.
These include ensuring that there are effective counter-terrorism laws and that measures are being taken to remove extremist content from social media.
"All action on this issue must be consistent with principles of a free, open and secure internet, without compromising human rights and fundamental freedoms, including freedom of expression," it says.
"It must also recognise the internet's ability to act as a force for good, including by promoting innovation and economic development and fostering inclusive societies."
Countries including Australia, Germany, India and Sweden have backed the action, alongside tech giants Facebook, Amazon, Google, Twitter and Microsoft.
In its statement the White House said it supported the Christchurch Call's "overall goals" but was "not currently in a position to join the endorsement".
Without directly highlighting specific issues with the initiative, the statement stressed the need to protect free speech.