Republican Donald Trump called U.S. President Barack Obama and Democrat Hillary Clinton the "co-founders" of Islamic State, ratcheting up his assertion that they are responsible for the rise of the militant group and sparking renewed criticism of his leadership ability.
Clinton's campaign called the statements a "false claim."
"This is another example of Donald Trump trash-talking the United States," senior policy adviser Jake Sullivan said in a statement.
"What's remarkable about Trump's comments is that once again, he's echoing the talking points of (Russian President Vladimir) Putin and our adversaries to attack American leaders and American interests, while failing to offer any serious plans to confront terrorism or make this country more secure," Sullivan said.
The White House declined to comment on Trump's claim.
Trump, coming off a troubled week in which fellow Republicans pressed him to stop quibbling with members of his own party and refocus his campaign attacks on Clinton and Obama, took his criticism to a new level by saying a sitting U.S. president created a militant group determined to kill Americans and other Westerners.
Trump has sought to bolster a perception of fear, delivering a campaign message that the United States is less safe, that Democrats are to blame and that he is the only one capable of restoring security.
Appearing in Miami Beach, Florida, on Thursday morning, Trump repeated his attack for the third time, saying the U.S. government "has unleashed ISIS."
"In fact, I think we'll give Hillary Clinton ... most valuable player," Trump said, attacking the Democratic candidate for her time as secretary of state from 2009-2013. "ISIS will hand her the most valuable player award. Her only competition is President Barack Obama."
Trump first made the assertion in a speech on Wednesday night in Florida, saying, "I call them co-founders" of Islamic State.
In an interview on Thursday morning, Trump defended the remarks.
"Is there something wrong with saying that?" Trump told CNBC. "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."
Clinton spokesman Jesse Lehrich, in response to Trump's initial remarks, pointed to U.S. advances against the militant group in Libya this week. "FYI - U.S.-backed militias retook ISIS's stronghold in Libya today thanks to Obama-authorized air strikes," he said in a tweet late on Wednesday.
The Democratic National Committee lambasted Trump's remarks as "outrageous, unhinged and patently false."
"Donald Trump should apologize for his outrageous, unhinged and patently false suggestions on the founding of ISIS," the DNC said in a statement. "This is yet another out of control statement by a candidate who is unraveling before our very eyes."
Trump's remarks followed a troubled week for the Republican candidate in which he drew strong criticism for a persistent confrontation with the family of a Muslim American soldier who was killed in Iraq and for his initial refusal to support prominent Republican congressional candidates in their primary races.
He also prompted strong criticism from Democrats for remarks on gun rights that they said were a call for violence against Clinton. Trump repeatedly denied he was inciting violence.
Time magazine reported on Thursday that Republican Chairman Reince Priebus had told Trump that party funds and staff would be redirected to help congressional candidates if his campaign continued to falter. Republican Party spokesman Sean Spicer declared the report "not true."
Recent opinion polls have shown Trump losing ground to Clinton, a former U.S. senator and first lady, in the race for the Nov. 8 election. An average of polls by RealClearPolitics has Clinton 7.7 percentage points ahead, at 48 percent to his 40.3 percent.