Donald Trump has directly called Hillary Clinton "the devil" as the Republican steps up his increasingly heated rhetoric against his Democrat opponent.

Speaking at a rally in Pennsylvania last night, he criticised Clinton's democratic rival for the presidential nomination Bernie Sanders, saying he "made a deal with the devil". He added of Clinton: "She's the devil".

At an earlier rally in Ohio, Trump said that he feared the election in November "is going to be rigged".

The comments come after a row over Trump's attack on the parents of US Army Captain Humayun Khan, who was killed by a car bomb in 2004 in Iraq.

Senator John McCain, a veteran of the Vietnam War and former Republican candidate for president, became the latest in a chorus of cross-party criticism of Trump, saying he does not have "unfettered licence to defame the best among us".

The dispute began last Thursday night when the late soldier's father Khizr Khan addressed the Democratic National Convention (DNC) with his wife, Ghazala, standing at his side.

Khan spoke emotionally of the sacrifice his son had made for the country as an American Muslim, specifically criticising Trump's proposal to ban Muslims from entering the country.

In response to the speech, Trump suggested that Ghazala Khan might not have been "allowed" to speak, implying her silence reflected restrictions placed on women by some conservative Muslims. "She had nothing to say, maybe she was not allowed to speak, you tell me," he added on Sunday.

Ghazala Khan wrote an opinion piece in the Washington Post on Sunday saying that she had remained silent during her husband's remarks because she was worried she would be too emotional to speak.

The Khans then conducted several interviews, discussing the support they have received since Trump's attacks.

But Trump took to Twitter to say: "This story is not about Mr Khan, who is all over the place doing interviews, but rather RADICAL ISLAMIC TERRORISM and the US. Get smart!"

Clinton has said Trump has treated the parents as scapegoats, while leading Republicans, including House Speaker Paul Ryan and Senate Majority Leader Mitch McConnell, issued statements in support of the family.

Last year, Trump claimed that McCain - who was taken prisoner for five years during the Vietnam War - was not a hero since he had been captured.

In a statement last night, McCain said Trump's latest views do not represent "our Republican Party."