U.S. Democratic presidential front-runner Hillary Clinton made a revealing slip of the tongue when she branded Republican candidate Donald Trump as "ISIS best recruiter" during the Democratic presidential debate on Saturday, commentators said.

In a speech last month in New York, Clinton said Muslims "have nothing whatsoever to do with terrorism," WND reported.

But radio host Rush Limbaugh said Monday that Clinton calling Trump "ISIS' best recruiter" makes no sense if indeed the terrorist group Islamic State is a total perversion of Islam as Clinton claims.

In Saturday's debate in Manchester, New Hampshire, Clinton said: "A lot of people are understandably reacting out of fear and anxiety about what they're seeing [with terrorist attacks], and Mr. Trump has a great capacity to use bluster and bigotry to inflame people and to make them think there are easy answers to very complex questions. He is becoming ISIS' best recruiter. They are going to people showing videos of Donald Trump insulting Islam and Muslims in order to recruit more radical jihadists."

A Clinton spokeswoman later said they could not cite a specific ISIS video using Trump as a recruitment tool.

Limbaugh said Clinton made a huge blunder, saying her comments make her an "abject fool" for inadvertently supporting Trump's call for a halt to Muslim immigration.

"Now, stop and think of that for a second. What is she saying? She's basically saying that Donald Trump saying there should be a moratorium on Muslims can drive them to terrorism," said Limbaugh. "Well, then isn't Trump right? I mean, if all it takes is suggesting there's a moratorium for a while on Muslims entering the country until we get a handle here on who's coming in, who's already here, what their plans are, and if that's going to cause them to go join ISIS, isn't Trump right? [...] But then Islam's a religion of peace. How can people of a religion of peace end up joining a terror group when Islam isn't about terror?"

Politifact rated Clinton's claim as "false," saying that while experts say that ISIS could be circulating Trump's sentiments to bolster their arguments that the West is at war with Islam, there's no evidence to date that Trump's rhetoric has been used in recruiting videos, NBC News said.

Trump wasted no time in demanding an apology from Clinton during an interview Monday on NBC's "The Today Show."

"I will demand an apology from Hillary," said the Republican presidential front-runner. "She should apologise. She lies about emails, she lies about Whitewater, she lies about everything. She will be a disaster as president of the United States."

A Clinton spokesman, however, said "hell, no," the presumptive Democratic presidential nominee won't apologise to Trump, NBC News said.

"Hillary Clinton will not be apologising to Donald Trump for correctly pointing out how his hateful rhetoric only helps ISIS recruit more terrorists," said spokesman Brian Fallon in a statement.