Hillary Clinton said yesterday (Thursday) that Isil's leaders were praying for Donald Trump to win the US election.
Islamic State of Iraq and the Levant would view a victory for Mr Trump as a "gift" from Allah, and believed it would provide "more motivation to every jihadi", the Democratic nominee said.
Mrs Clinton made the comments during a rare press conference, in response to a question about a Time magazine article which included quotes from Isil supporters openly rooting for Mr Trump.
"They have said that they hope that he is the president because it would give even more motivation to every jihadi," she said.
"Someone who has insulted Muslims, has insulted a Gold Star family of an incredibly heroic Captain Khan, someone who has said he doesn't want to let Muslims around the world even come to our country - that is a gift for Isis," she added, using an alternative name for the group.
The comments came a day after Mrs Clinton and Mr Trump took part in a televised foreign policy forum.
In it, Mr Trump praised Vladimir Putin for his high approval ratings and his "very strong control" over Russia, comparing him favourably to Barack Obama, the US president.
Mr Trump said he felt he could get along with Mr Putin, and was glad to have received a compliment from him.
"Well I think when he called me brilliant, I'll take the compliment, OK?" Mr Trump said.
"It's a very different system and I don't happen to like the system, but certainly, in that system, he's been a leader, far more than our president has been a leader."
Mrs Clinton described his remarks as "astonishing" during a rally yesterday in North Carolina.
"It's not just unpatriotic, it's not just insulting to the office and the man who holds the office," she said, defending Mr Obama with visible anger. "It is scary. It is dangerous. It actually suggests that he will let Putin do whatever Putin wants and then make excuses for him."
Mrs Clinton also excoriated Mr Trump for a remark that America's generals had been "reduced to rubble" in recent years, and his suggestion that he would remove the top brass and replace them with leaders of his choosing.
Mr Trump, for his part, said Mrs Clinton's performance in a forum on Wednesday showed her to be "unfit to serve" as president.
"Hillary Clinton answered email questions differently last night than she has in the past," he said. "She is totally confused."
Donald Trump Jr, the candidate's son, shared a post yesterday from Infowars, a website known for spreading conspiracy theories, which alleged that Mrs Clinton had been wearing an earpiece during the forum so her staff could help her respond to questions.
The theory was denied by the Clinton campaign.
Mr Trump appears to be gaining ground. The four most recent national polls have all showed the Republican candidate within two percentage points of Mrs Clinton.