Media playback is unsupported on your device Media caption A history of calling Barack Obama a Muslim

Donald Trump is taking flak from fellow Republicans for not correcting a supporter who said US President Barack Obama was a Muslim and not American.

The businessman, who is the frontrunner for the Republican nomination for president, has been strongly criticised by two of his Republican rivals.

South Carolina Senator Lindsey Graham said Mr Trump acted inappropriately and he should apologise.

Mr Trump has cancelled his appearance at a big Republican event on Friday.

His campaign team said he was pulling out of the Heritage Foundation because of a "significant business transaction" that needed his attention.

The criticism has been piling up since a man at Mr Trump's rally in New Hampshire on Thursday night prefaced a question by saying Mr Obama was a Muslim and "not even an American".

The supporter went on to say: "We have a problem in this country - it's called Muslims."

Media playback is unsupported on your device Media caption Republican Donald Trump sought to laugh off the comments

Mr Trump let it go unchallenged and within a few hours, Democratic frontrunner Hillary Clinton said his failure to denounce "hateful rhetoric" was "disturbing and wrong".

And on Friday his Republican competitors for the nomination waded into the row.

"He's playing into this hateful narrative and he has to set it right," said Mr Graham, who said he would never question the president's faith or patriotism.

Leaders have an "obligation" to correct such statements, said New Jersey Governor Chris Christie.

"If somebody at one of my town hall meetings said that, I would correct them and I would say, 'The president's a Christian and he was born in this country. Those two things are self-evident,'" said Mr Christie.

The White House also responded. Spokesman Josh Earnest said: "Mr Trump isn't the first Republican politician to countenance these views to gain votes."

Image copyright Twitter Image caption Democratic contender Martin O'Malley also had strong words for Trump

Corey Lewandowski, Mr Trump's campaign manager, sought to play down the incident, telling US media that "all he heard was a question about training camps".

President Obama, who has spoken openly about his Christian faith, was born to an American mother and Kenyan father in Hawaii.

But Mr Trump has been one of the leading sceptics, challenging Mr Obama in 2011 to produce his birth certificate to disprove rumours that he was born in Kenya, which the president did.

With more than a year until polling day, the businessman is ahead of his Republican rivals in the polls despite having no political experience.

2016 hopefuls

Image copyright AP Image caption Clockwise from top left: Donald Trump, Jeb Bush, Scott Walker, Marco Rubio, Bernie Sanders, Hillary Clinton

The early Republican frontrunner is Donald Trump

Hillary Clinton will have learnt much from her failed campaign of 2008

Florida senator Marco Rubio lost some right-wing fans by backing a bipartisan immigration reform package

Wisconsin governor Scott Walker appeals to both the Republican establishment and the Tea Party

Libertarian Rand Paul has his supporters - and enemies - among Republicans

Veteran congressman Bernie Sanders is drawing huge crowds at his rallies

Meet all of the 2016 hopefuls