DES MOINES, United States (AFP)— Republican presidential frontrunner Donald Trump said Saturday he was not morally obligated to defend Barack Obama after he let pass unchallenged a questioner’s assertion that the US leader is a Muslim.

In a rapid-fire series of tweets, the billionaire real estate mogul responded to a barrage of criticism of his handling of the incident, which came from Democrats and one fellow Republican candidate, New Jersey Governor Chris Christie.

“Am I morally obligated to defend the president every time somebody says something bad or controversial about him? I don’t think so!” Trump said.

Trump, who helped fuel a “birther” movement in 2011 by repeatedly demanding Obama prove he wasn’t born in Kenya, appeared to encourage a man at a campaign stop in New Hampshire Thursday after he made the incorrect assertion about Obama’s faith.

“We have a problem in this country, it’s called Muslims. We know our current president is one, you know he’s not even an American,” the unidentified questioner said.

Trump chuckled and interrupted him to say, “We need this question. This is the first question.”

Putting himself in Trump’s shoes, Christie said he would have corrected the questioner and said: “No, the president’s a Christian and he was born in this country.”

Hillary Clinton, the leading Democratic candidate, called Trump’s handling of the incident “disturbing, and just plain wrong,” while White House spokesman Josh Earnest said it was no surprise because “the people who hold these views are part of Mr Trump’s base”.

But Trump did not back down, saying Saturday it was the first time in his life he had caused a controversy “by NOT saying something”.

“If someone made a nasty or controversial statement about me to the president, do you really think he would come to my rescue? No chance!” he said in his Twitter barrage.

“If I would have challenged the man, the media would have accused me of interfering with that man’s right of free speech. A no win situation!”

The belief that Obama is a Muslim is not uncommon, particularly among Republicans. A recent CNN/ORC poll found that 29 per cent of Americans believe it, as do 43 per cent of Republicans.

Trump, who cancelled a campaign stop in South Carolina Friday after the flap broke, was scheduled to make two campaign appearances Saturday in Iowa, the first state to vote in the Republican contest.