He did not elaborate on that claim.
The controversial Filipino leader, whose crackdown on his country's long-standing drug problems involved killing hundreds of suspected drug dealers, told soldiers during a speech last week in Mindanao's province of Zamboanga del Sur that in "three to seven years," the country will be plagued with "the ISIS disease."
"They are not armed, but they are here for indoctrination," Duterte said, according to the Philippine Inquirer. "That's what I'm afraid of."
During his speech Monday, Duterte said the terrorist group does have not any political ideology and does not have "any concept of what God really is."
"You maim people, you kill them," Duterte said. "People who refuse to have sex with them, they simply burn them."
He also warned that confronting the terrorist group will "bring the worst out of me," adding that he will never allow his country to be destroyed by terrorism, even if it meant being forced out of office.
"If you can do it, I can do it 10 times better than you, definitely," Duterte said. "I'll put at stake my honor, my life and the presidency."
Last month, Reuters reported that a 20-minute video showed Southeast Asian militants who claimed to be ISIS fighters urging Muslims to unite under Abu Abdullah, leader of the Philippine militant group Abu Sayyaf.
"If you cannot go to [Syria], join up and go to the Philippines," a man identified as Mohd Rafi Udin, a Malaysian militant, said in Malay.
The Philippines' tough-talking leader drew international scorn in April for remarks about an Australian missionary who was raped and murdered in Davao City jail in 1989, when Duterte was the city's mayor.
Duterte, then a presidential candidate, said the woman was "so beautiful" that he, as the mayor, "should have been first," according to CNN. The 71-year-old Duterte refused to apologize for the comments.
Duterte's brash talk has also earned him a reputation as Asia's Donald Trump.
During his campaign for president and again after his election in May, the former prosecutor nicknamed "The Punisher" said that journalists killed on the job were corrupt and that they deserved it.
He also advocated for extrajudicial killing of criminals, saying members of the public can kill them if they resist or threaten citizens with weapons.
"Shoot him and I'll give you a medal," Duterte said.
More recently, Duterte used a homophobic slur in describing Philip Goldberg, the U.S. ambassador to the Philippines, as well as calling him a "son of a whore."
Duterte's tough talk became more than just words shortly after he began his presidency.
“My order is shoot to kill you. I don’t care about human rights, you better believe me,” Duterte said earlier this month.
The killings alarmed human rights groups and prompted protests from left-wing activists. The head of the Senate's committee of justice and human rights also said that an investigation into the killings will begin next week.
Duterte's war on drugs also is likely to worsen the country's problem with jail overcrowding "as hundreds of thousands of new detainees are crammed into detention facilities," Carlos Conde, a researcher for Human Rights Watch's Asia division, told The Washington Post.
Conde said the country's jail system already does not have adequate resources to handle detainees, the majority of whom face drug charges and are being held without bail as they await trial — and that’s even before Duterte’s proposed “war on terror” begins.