Republican presidential candidate Donald Trump has called US President Barack Obama and Democratic candidate Hillary Clinton “co-founders” of Islamic State, igniting fresh criticism of his inflammatory campaign style.

The New York tycoon has previously attacked Obama and Clinton, secretary of state from 2009-13, for how the United States pulled out of Iraq after the war, saying it helped create the militant Islamist group that has seized swaths of Iraq and Syria.

The idea that a sitting US president created a militant group determined to kill Americans and other Westerners took that line of attack to a new level.

Trump first made the assertion in a speech on Wednesday night in Florida. He repeated it in an interview on Thursday morning with CNBC.

His remarks followed a troubled week for the Republican candidate. Party leaders urged Trump to focus on the campaign to beat Clinton after he drew strong criticism for a persistent confrontation with the family of Muslim American soldier who died in Iraq and for his initial refusal to support prominent Republican congressional candidates in their primary races.

Recent opinion polls have shown Trump losing ground to Clinton, a former US senator and first lady, in the race for the November 8 election.

An average of polls by RealClearPolitics has Clinton 7.7 percentage points ahead, at 48 per cent to his 40.3 per cent.

If Trump doesn’t win the election he will go back to a “good life”. Photo: AAP

“He (Obama) was the founder of ISIS. And so was she. I mean I call them co-founders,” said Trump, who says he opposed the Iraq war.

“He shouldn’t have gotten out the way he got out. It was a disaster, what he did,” he told CNBC.

Obama had opposed the Iraq war and campaigned for the White House in 2008 on a promise to end it. The United States pulled out combat troops in 2011.

Islamic State in Iraq and Syria, also known as ISIS or ISIL, had its roots in the al-Qaeda insurgency that arose after the US led an invasion of Iraq in 2003.

Clinton spokesman Jesse Lehrich, in response to Trump’s comments, pointed to US advances against the militant group in Libya this week.

“FYI – US-backed militias retook ISIS’s stronghold in Libya today thanks to Obama-authorised air strikes,” he said in a tweet.

When is the media going to talk about Hillary's policies that have gotten people killed, like Libya, open borders, and maybe her emails?

— Donald J. Trump (@realDonaldTrump) August 10, 2016

Trump did not back down, asking on CNBC: “Is there something wrong with saying that? Why – are people complaining that I said he was the founder of ISIS? All I do is tell the truth, I’m a truth teller.”

Many Republicans have urged Trump to change tactics and focus on the economy.

US Representative Sean Duffy, a Republican from Wisconsin who backs Trump, said Obama and Clinton did not found Islamic State and urged Trump to stay on message.

“Stay on script. Don’t go off script. Read your teleprompter and you’re going to be fine,” Duffy said on MSNBC.

Trump bristles at the notion he should change. “I don’t think I’ve made too many errors,” he told CNBC.

If his style costs him the election in 90 days, he goes back to a good life, he said.

“It’s not what I’m looking to do – I think we’re going to have a victory but we’ll see,” Trump added.