Nineteen US states have introduced bills that would curb freedom of expression and the right to protest since Donald Trump's election as president, an "alarming and undemocratic" trend, UN human rights investigators say.

Maina Kiai and David Kaye, independent UN experts on freedom of peaceful assembly and expression, said on Thursday the moves were incompatible with international human rights law.

"The trend also threatens to jeopardise one of the United States' constitutional pillars: free speech," they said in a statement, calling for action to reverse such legislation.

"From the Black Lives Matter movement, to the environmental and Native American movements in opposition to the Dakota Access oil pipeline, and the Women's Marches, individuals and organisations across [American] society have mobilised in peaceful protests," Kiai and Kaye said.

The UN experts said it was Americans' fundamental right to protest, but the bills in Republican-governed states such as Indiana, Arkansas, Florida, Georgia, Indiana, Iowa, Michigan and Missouri sought to stop them exercising that right.

Several bills proposed in Colorado, North Dakota, and Oklahoma targeted opponents of the Dakota Access Pipeline in North Dakota and would have "a chilling effect on environmental protesters", the experts said.

Last month, dozens of armed US law enforcement officers swept through a protest camp near the site of the pipeline, clearing the gathering that for months served as a base of opposition to the multi-billion-dollar project.

In Missouri, a bill proposed a seven-year prison term for "unlawful obstruction of traffic", while the Minnesota bill would criminalise peaceful protesters for participating in demonstrations that subsequently turned violent.

The UN experts said there was no such thing as a violent protest, only violent protesters.

"One person's decision to resort to violence does not strip other protesters of their right to freedom of peaceful assembly," Kaye and Kiai said.

Supporters of the US state legislative action say it sums up the frustration some people feel about protests that get in the way of daily lives, and reflects a wish to maintain public safety.

Free speech advocates say the bills are worrying, seeing them as opening the way to criminalising peaceful protests.