Washington DC - 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 organization," 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."

Trump 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.

Kirk, who was a close ally of Trump, was shot dead last week during a speaking event on a Utah university campus.

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Authorities have formally charged 22-year-old Tyler Robinson with the murder, alleging that he used a rifle to shoot Kirk with a single bullet to the neck from a rooftop.

Earlier this week, Utah Governor Spencer Cox said that "there clearly was a leftist ideology with this – with this assassin."

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

While federal law enforcement includes combating domestic terrorism under its purview, the US does not have a list of designated "domestic terrorist organizations."