Trump hits back at Obama, says Clinton 'unfit' to lead

US Republican presidential nominee Donald Trump hit back at President Barack Obama on Tuesday evening after he was accused of being unfit to take office.

Obama criticised the billionaire candidate earlier in the day for criticising the Muslim family of a US Army captain who was killed while serving in Iraq.

The spat between Trump and the Khan family started last week when Khizr Khan, the father of the dead soldier, attacked the Republican nominee for calling for a ban on Muslims entering the US.

Trump responded to Khan by accusing him of silencing his wife Ghazala, who did not speak while standing beside her husband at the Democratic National Convention.

Ghazala Khan later denied Trump's claim, saying that she chose not to speak because she was overcome by emotion when she saw her son's picture.

Trump's attitude towards the Gold Star family triggered a wave of criticism, even from those supporting his presidential campaign, including Speaker of the House of Representatives Paul Ryan, Senate Republican leader Mitch McConnell, Senator John McCain.

Referring to the spat, President Obama told reporters in a White House news conference on Tuesday that Trump is unfit to replace him as president.

"The notion that he would attack a Gold Star family that had made such extraordinary sacrifices on behalf of our country, the fact that he doesn't appear to have basic knowledge around critical issues in Europe, in the Middle East, in Asia means that he's woefully unprepared to do this job," Obama said.

The outgoing president, whose term runs out this year, also slammed Trump's Republican backers who continue to support him despite acknowledging his behaviour as unacceptable.

"What does this say about your party that this is your standard-bearer?" said Obama, who last week officially endorsed Democratic presidential nominee Hillary Clinton's campaign.

"There has to come a point at which you say, somebody who makes those kinds of statements doesn't have the judgment, the temperament, the understanding to occupy the most powerful position in the world," he added.

But Trump wasted no time in responding to Obama's criticism, saying that Clinton is the one who is unfit to lead.

"They have betrayed our security and our workers, and Hillary Clinton has proven herself unfit to serve in any government office," Trump said.

Clinton is ‘the devil'

Trump's clash with Obama came just a day after the Republican nominee called his challenger Clinton "the devil."

"She's the devil. He made a deal with the devil," Trump said referring to the agreement between Vermont senator Bernie Sanders and Clinton.

"They are talking about Bernie cause he made a bad deal. He should have not made a deal. He would have gone down as doing something really important," Trump said at a town hall event in Columbus, Ohio.

"Once he made that deal, and believe me he has buyer's remorse. You know this guy has buyer's remorse."

Trump was speaking about a symbolic show of party unity during the Democratic National Convention when Sanders told the chairwoman from the convention floor that Hillary Clinton should be selected as the party's nominee.

The Republican nominee also told supporters he thought the November election might be "rigged".

"I'm afraid the election is going to be rigged, I have to be honest," he said at the event, before later repeating the claim on Fox News. "I hope the Republicans are watching closely or it's going to be taken away from us."