'If I lose and if it's a fair election, I will concede defeat': Trump

US Republican presidential contender Donald Trump has said he would be prepared to concede defeat after Tuesday's vote "if it's a fair election", while again raising concerns about the use of electronic voting machines.

"If I lose an election, if it's a fair election, I would be the first one to acknowledge it. So far I think it's been fair," Trump, repeating a caveat that he has used many times on the campaign trail, told reporters after voting in Florida on Tuesday.

Wearing a red "Make America Great Again" cap, he reiterated his previous criticism of electronic voting machines, suggesting they were less secure than paper ballots and would delay the outcome being known.

"They spend all this money on machines, If they would use paper ballots, voter ID, proof of citizenship, and one-day voting, it would all be over by 10 o'clock in the evening. It's crazy," he told reporters in West Palm Beach.

He added: "Do you know that paper is more sophisticated now than computers? If it's watermarked paper you cannot. It's unbelievable what happens with it. There's nothing you can do to cheat."

Asked about fears of unrest after the election and whether he would call on supporters to avoid violence, he criticised the question.

"I don't have to tell them that, that there'll be no violence. Of course, there'll be no violence. My supporters are not violent people," Trump said.