Donald Trump backtracked Friday from his assertion that President Barack Obama and Hillary Clinton founded the Islamic State group, saying he was just being sarcastic.

As he often does, the Republican presidential nominee accused the news media of misconstruing something he said.

In this case he targeted CNN, although his comments on the jihadist group and the president were picked up across the news spectrum.

"Ratings challenged @CNN reports so seriously that I call President Obama (and Clinton) "the founder" of ISIS, & MVP. THEY DON'T GET SARCASM?," Trump wrote in a tweet.

Trump first made the accusation Wednesday at a rally in Florida, and repeated it in interviews Thursday.

He appeared to be mimicking the argument that the US troop withdrawal from Iraq under Obama, with Clinton serving as secretary of state, created a vacuum that allowed the Islamic State group to emerge and flourish in Iraq and Syria.

But Trump did not explain fully what he meant.

He also said he considered Clinton, his Democratic rival for the presidency, to be the co-founder of the Islamic State group.

The Clinton team responded Thursday by calling the assertion outlandish.

"Anyone willing to sink so low, so often should never be allowed to serve as our commander-in-chief," Clinton wrote in a tweet.

And the Democratic National Committee on called on the real estate mogul to "apologize for his outrageous, unhinged and patently false suggestions."

Trump tends to stand pat by his often freewheeling accusations and assertions.

However last week he did acknowledge an error, which was very rare for him.

Trump acknowledged August 5 he was wrong in claiming to have seen secret Iranian footage of $400 million in cash being delivered to Tehran as payment for the release of US prisoners.

Trump raised eyebrows when he made that claim and gave many details of what he said he saw in the film.

But that widely viewed footage is believed instead to show the moment in January when three of five American prisoners freed by Iran get off a plane in Geneva.

In a tweet last Friday, Trump said this is indeed what he saw. (AFP)