A gunman lays on his stomach on a rooftop with a rifle in hand, then screams ring out, a video published by US outlet TMZ on Saturday shows.

US presidential candidate Donald Trump was shot in the ear in an assassination attempt by a gunman at a campaign rally Saturday in Butler, Pennsylvania.

The FBI identified the gunman, who was shot dead by the US Secret Service, as Thomas Matthew Crooks, a 20-year-old from the US state's Bethel Park, according to US media.

The video published by TMZ shows the alleged shooter laying on his belly on a rooftop and aiming a rifle.

"The guy has longer brown hair, and he seems to be wearing a grey shirt/khaki pants -- and as you can see, he's carefully trying pinpoint a target from afar before pulling the trigger," TMZ reported.

The man is not shown firing in the published footage. But a quick succession of gunshots is heard followed by screams from people off-camera.

"He's turning this way, be careful guys," a man is heard saying.

"Somebody got shot," another voice says. "He's right there, laying down."

"Dude, that's exactly why we need... Trump here," another man is heard saying.

"Secret Service don't (mess) around," another man says.

A second video published by TMZ shows shaky footage of the same rooftop and a blurred body.

"You can see the guy there," a person off-camera says. "I think they hit him because the guy, he looks dead."

Another video posted by a TikTok user shows Trump touching his right ear just after multiple bangs are heard.

In the same frame, a security force sniper, positioned on a roof behind the Republican presidential candidate, is seen taking aim with a rifle mounted on a stand.

AFP did not independently verify the videos, and it is unclear whether the sniper fired the weapon.

Trump then crouches down behind the lectern as Secret Service agents move in around him. Screams are heard in the crowd.

He is later escorted off stage by agents.