His latest tweet echoes a post from late July in which he similarly stated that he was considering labelling Antifa as terrorists.
Right-wing rallies will take place on Saturday in Portland, Oregon, and left-wing activists from Antifa intend to stage rival rallies. Oregon police are taking efforts to ensure safety and an additional 1,000 police officers will be deployed on the street.
Political opponents previously repeatedly accused Trump of sympathising with the right-wing, but the US president denies this.
Antifa is a left-wing organisation classified as domestic terrorists by the FBI and the DHS. The group’s members are reported to have assaulted their right-wing rivals.