'Hillary's the devil': Trump

Republican presidential nominee Donald Trump has suggested that the November election could be "rigged".

He told a rally in Columbus, Ohio, that he had heard "more and more" that the contest would be unfair. He offered no immediate evidence.

At another event he called Democratic rival Hillary Clinton "the devil". Trump has come under fire from across the political divide for remarks he made about the parents of a US Muslim soldier killed in action.

On the forthcoming vote, he told supporters "I'm afraid the election is going to be rigged, I have to be honest".

He later repeated the claim on Fox News, adding "I hope the Republicans are watching closely or it's going to be taken away from us." Trump has made similar comments before in relation to the Democratic race, suggesting the party fixed its system to favour nominee Hillary Clinton over her challenger Bernie Sanders.

Meanwhile, US President Barack Obama yesterday called out Republican leaders on their continued support for their party's White House nominee Donald Trump, given some of the real estate mogul's incendiary statements.

"This isn't a situation where you have an episodic gaffe. This is daily and weekly where they are distancing themselves from statements he's making," Obama said of the party's top brass including House Speaker Paul Ryan and Senator John McCain.

"There has to be a point in which you say: 'This is not somebody I can support for president of the United States, even if he purports to be a member of my party.'"

At another rally in Pennsylvania, Trump took the unprecedented step of directly calling Clinton "the devil".

He attacked Sanders for capitulating in the Democratic race, saying he "made a deal with the devil. She's the devil."

Democrats and Republicans alike have condemned Trump for his remarks about the parents of US Army Capt Humayun Khan, who was killed by a car bomb in 2004 in Iraq, at the age of 27.

Former Republican presidential candidate John McCain became the latest senior figure in the party to criticise Trump for his attacks.

Senator McCain, a veteran of the Vietnam War, said in a strongly worded statement that Trump did not have "unfettered licence to defame the best among us".

In another development, American billionaire businessman Warren Buffett challenged Trump to release his tax returns. Trump has said that they cannot be made public until the financial authorities have completed an audit.

But Buffett said there were no rules against showing tax returns and allowing people to ask questions about them.