A witness to Saturday's shooting at a Donald Trump rally told how a man next to him was shot dead during the apparent assassination attempt on the Republican presidential candidate.

"I heard several gunshots. The man beside me suffered a gun shot to the head, was instantly killed (and) fell to the bottom of the bleachers. Another woman looked like she got hit in the forearm or hand," the man, whose name was only given as Joseph, told NBC News.

He said it seemed the victim was "in the way of the shots between whoever was shooting the gun and the president."

A second witness, an unnamed man who said he was an emergency doctor, described trying to help one of the victims. 

"Somebody over there was screaming he's been shot, he's been shot, so I made my way over, I said I'm an emergency department physician, let me help you," said the man, who was wearing a white USA T-shirt stained with blood and a red MAGA hat.

"The guy had spun around, was jammed between the benches. He had a headshot here," he said, indicating a spot on his head.

"There's lots of blood and he had brain matter," the man continued in comments that spread widely through US media.

Trump said he was hit in the ear but was not seriously injured by the gunman, who was killed after the incident. 

Two other spectators were critically injured, the Secret Service said.