The announcement, made by Trump on Twitter, comes amid violent protests across the US about police brutality following the death of a black man in Minneapolis seen on video gasping for breath as a white police officer knelt on his neck.

Several top officials from the Trump administration, including US Attorney General William Barr, have blamed Antifa and other "agitators" for taking over the protests in US cities.

"The violence instigated and carried out by Antifa and other similar groups in connection with the rioting is domestic terrorism and will be treated accordingly," Barr said in a statement on Sunday following the president's tweet.

It is not clear how many, if any, of the protesters participating in demonstrations across the country are from Antifa, which experts note is not an organisation but rather an amorphous movement.

John Harrington, the head of Minnesota's Department of Public Safety, told a news conference on Sunday that about 20 per cent of Saturday's arrest records were for people out-of-state, though he did not yet have the total for Saturday night's arrests.

Trump's tweet on Sunday is not the first time the president has described Antifa as a terrorist group. Other conservative politicians, such as Texas Senator Ted Cruz, have made similar statements.

It is unclear whether the Trump administration is seriously pursuing the designation through formal channels, which would typically require co-ordination across multiple federal agencies. Experts say Trump lacks the legal authority to do so.

"Terrorism is an inherently political label, easily abused and misused," said ACLU National Security Project Director Hina Shamsi.

Mary McCord, a former senior Justice Department official, said "no current legal authority exists for designating domestic organisations as terrorist organisations".

"Any attempt at such a designation would raise significant First Amendment concerns," added McCord, who previously served in the Trump administration.

Australian Associated Press