US regulators voted Thursday to roll back so-called "net neutrality" rules which required internet providers to treat all traffic equally, a move opponents say would curb online freedom.

The Federal Communications Commission, in a three-to-two vote, adopted a proposal by Republican-appointed chairman Ajit Pai, who said his plan would scrap "heavy-handed" rules adopted in 2015 which he argued discouraged investment and innovation.

The vote capped a heated partisan debate and is just the latest in a battle over more than a decade on rules governing internet service providers in the courts and the FCC.

Democratic member Mignon Clyburn, one of the two dissenters, charged that the agency was "handing the keys to the internet" to "a handful of multibillion dollar corporations."

Immediately following the vote, officials from two states and others vowed to challenge the FCC action in court.