US Republican presidential nominee Donald Trump has claimed that the US presidential elections in November might be "rigged".

While rallying his supporters in Pennsylvania last night, Trump said that he had been hearing "more and more" that the election may not be contested fairly, though he did not elaborate further.

He made the claim after first suggesting that US Senator Bernie Sanders never had a chance of winning the Democratic presidential nomination because the party's primary contest had been "rigged" in favour of Hillary Clinton.

He also berated Sanders decision to endorse Clinton, suggesting he may have "buyers remorse", before adding: "He made a deal with the devil. She's the devil."

These latest comments come in the wake of a public feud between Donald Trump and the parents of a Muslim US soldier killed in action by a suicide bomber in Iraq in 2004.

Khizr Khan and his wife, Ghazala, appeared on stage at last week's Democratic convention, where he said Trump had "sacrificed nothing and no one" for his country.

Khizr Khan & Ghazala Khan | Image: J. Scott Applewhite / AP/Press Association Images

The Republican presidential candidate hit back, claiming he had given up a lot for his businesses, and sparked outrage by implying Mrs Khan did not speak while standing alongside her husband because she was forbidden as a Muslim woman.

He has since come under fire from veteran groups, relatives of soldiers killed in action, President Obama and even some Republicans.

Former Republican presidential candidate John McCain, who ran unsuccessfully for the White House in 2008, entered the row, denouncing the property tycoon's "disparaging" comments.

"I hope Americans understand that the remarks do not represent the views of our Republican Party, its officers or candidates," he said.

"While our party has bestowed upon him the nomination, it is not accompanied by unfettered license to defame those who are the best among us."

Writing in the Washington Post, Mrs Khan said she did not speak because she finds talking about her son's death extremely difficult.

"My husband asked me if I wanted to speak, but I told him I could not," she wrote.

In reference to Trump's comments, she said: "When Donald Trump is talking about Islam, he is ignorant."

"Donald Trump said he has made a lot of sacrifices. He doesn’t know what the word sacrifice means."

Additional reporting by IRN