Republican candidate and former US president Donald Trump on Sunday told the New York Post he was "supposed to be dead" after surviving an assassination attempt which he described as a "very surreal experience."

"I'm not supposed to be here, I'm supposed to be dead," Trump told the Post in an interview aboard his plane en route to Milwaukee for the Republican National Convention where he is set to be confirmed as the party's presidential candidate.

It was a "very surreal experience" he recounted with a white bandage covering his right ear, the paper said.

The 78-year-old Trump was hit in the ear by a gunman at a campaign rally on Saturday.

He was left with a bloodied face while a bystander was killed and two other people were wounded.

Trump told the Post he would have been dead had he not tilted his head slightly to the right to read a chart on illegal immigrants while addressing the rally.

"By luck or by God, many people are saying it's by God I'm still here," he said.

He praised the Secret Service agents for killing the shooter.

"They took him out with one shot right between the eyes," he said.

"They did a fantastic job," he added. "It's surreal for all of us."

The image of Trump raising a defiant fist as Secret Service agents bundled him away made front pages around the world and spread virally on social media.

"A lot of people say it's the most iconic photo they've ever seen," the former president told the Post, adding "They're right and I didn't die. Usually you have to die to have an iconic picture."

Trump said after the attempt on his life he was rewriting the speech he had prepared for the Republican convention.

He said he had "prepared an extremely tough speech" about Biden's "horrible administration. But I threw it away" for one he hopes will "unite our country."

"But I don't know if that's possible. People are very divided."