UN experts urged American lawmakers Thursday to stop pushing "undemocratic" anti-protest legislation, which they warned breached US and international law ensuring rights to freedom of expression and assembly.

Since the presidential election in November, lawmakers in at least 19 US states have introduced bills to restrict the rights of protesters, the United Nations rights experts warned.

And since January, when President Donald Trump took office, "a number of undemocratic bills have been proposed in state legislatures with the purpose or effect of criminalising peaceful protests," they said in a statement.

David Kaye, the UN's top expert on freedom of expression, and Maina Kiai, the expert on freedom of assembly, also voiced their concerns in a letter sent to the US mission in Geneva earlier this week.

When contacted by AFP, the mission could not immediately provide comment.

The two experts pointed out that the bills were coming at a time when the United States was seeing some of the largest and most frequent demonstrations in its history, including by the Black Lives Matter movement, Native Americans opposing the Dakota Access oil pipeline and massive women's marches.

"The bills, if enacted into law, would severely infringe upon the exercise of the rights to freedom of expression and freedom of peaceful assembly in ways that are incompatible with US obligations under international human rights law and with First Amendment protections," they said.

"The trend also threatens to jeopardise one of the United States' constitutional pillars: free speech," they added.

The experts pointed for instance to an Indiana Senate bill that would allow law enforcement to "use any means necessary to clear the roads of people unlawfully obstructing vehicular traffic".

They also highlighted a Missouri bill that proposed prison terms of up to seven years for "unlawful obstruction of traffic," and bills in Florida and Tennessee that would exempt drivers from liability if they accidentally hit and kill people demonstrating in the road.

They also balked at other legislation characterising protests as "violent".

"There can be no such thing in law as a violent protest, ... (only) violent protesters, who should be dealt with individually and appropriately by law enforcement," they said.