US President Donald Trump on Wednesday said he would designate "Antifa" – a shorthand term for "anti-fascist" used to describe diffuse far-left groups – as "a major terrorist organisation," a move he has threatened since his first term.

"I will also be strongly recommending that those funding ANTIFA be thoroughly investigated in accordance with the highest legal standards and practices," Trump wrote on Truth Social, calling Antifa "A SICK, DANGEROUS, RADICAL LEFT DISASTER."

It was not clear what legal weight Trump's proclamation carried. Antifa is a loosely organised ideological movement without a clear leadership structure or hierarchy, experts said.

Trump had on Monday threatened to go through with such a designation after senior White House official Stephen Miller vowed the administration would dismantle an alleged "vast domestic terror movement" that he linked to the killing of right-wing activist Charlie Kirk.

However, no evidence has emerged connecting the alleged shooter, 22-year-old Tyler Robinson, with any outside group. Questions also remain about his precise motives.

Trump since his first term has ascribed blame to Antifa for various actions he dislikes, from violence against police to conducting the US Capitol riot on January 6, 2021.

While federal law enforcement includes combating domestic terrorism under its purview, the United States does not have a list of designated "domestic terrorist organisations." (Agencies)