US President Donald Trump urges supporters to respond peacefully to the murder of right-wing activist Charlie Kirk, while the perpetrator remains at large more than 24 hours after the shooting, which has put an already divided America on edge.

Trump, who shortly after the killing angrily promised a massive response against the "radical left," said Kirk was "an advocate of nonviolence."

"That's how I would like to see people respond," he said, quoted by AFP.

Kirk, 31, a rising star on the Republican right who had attracted widespread youth support for Trump, was shot while speaking to a large crowd at Utah Valley University on September 10.

Trump reported "great progress" in the investigation. However, authorities confirmed that the shooter is still at large.

"Several leads are currently being investigated, but no suspect has been taken into custody," Utah law enforcement officials said after canceling a previously scheduled press conference.

"We are doing everything we can to find him, but we are not sure how far he has gone," FBI Special Agent Robert Bolus said earlier.

The FBI, which described the attack on Kirk as "targeted," released blurry photos of a young man it called a "potential shooter."

The images show a man wearing a black baseball cap, dark sunglasses, jeans, and a long-sleeved shirt with a design featuring the American flag.

A reward of up to $100,000 has been offered for information leading to his arrest.

Police believe the shooter fired a single shot from a rooftop 180 meters away, hitting Kirk in the neck.

A powerful bolt-action rifle was found in a wooded area. | BGNES