Washington (AFP) - A top US regulator, defending an effort to roll back so-called "net neutrality" rules, said Tuesday that large internet platforms represent the biggest threat to online freedom because they routinely block "content they don't like."

Federal Communications Commission chairman Ajit Pai delivered remarks days after unveiling a proposal to reverse a hotly contested 2015 rule requiring broadband firms to treat all online traffic equally.

Pai said internet platforms -- he singled out Twitter -- play a more significant role than broadband operators in determining what internet users see.

"Despite all the talk about the fear that broadband providers could decide what internet content consumers can see, recent experience shows that so-called edge providers are in fact deciding what content they see," Pai said.

"These providers routinely block or discriminate against content they don't like."

The blunt remarks appeared to confirm a tougher atmosphere in Washington for Silicon Valley firms after years of close ties.

Pai, appointed by President Donald Trump, offered an example of Twitter's decision to block a video by a Republican candidate "because it featured a pro-life message," referring to the politician's claim of the "sale of baby body parts."

He said Twitter "appears to have a double standard when it comes to suspending or de-verifying conservative users' accounts as opposed to those of liberal users," Pai said.

"This conduct is many things, but it isn't fighting for an open internet."

Pai said online platforms are "secretly editing certain users' comments" and "caving to repressive foreign governments' demands to block certain speech" which would be considered "repugnant" in the United States.

"In this way, edge providers are a much bigger actual threat to an open internet than broadband providers, especially when it comes to discrimination on the basis of viewpoint," Pai said.

The dispute over net neutrality has been the subject of several court battles, with backers arguing strong rules are needed to guard against powerful broadband firms like Comcast and AT&T acting as "gatekeepers" that can punish rivals.

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Pai said the debate on "net neutrality" appears driven by Silicon Valley firms' business interests.

"These companies want to place much tougher regulations on broadband providers than they are willing to have placed upon themselves," he said.

"They might cloak their advocacy in the public interest, but the real interest of these Internet giants is in using the regulatory process to cement their dominance in the internet economy."