"I don't really know how many bullets from my gun went inside their bodies," he said. "It happened. I cannot lie about it."
His confirmation, reported by the Philippine Star, followed boasts to business leaders on Monday that he killed criminal suspects as the Davao City mayor to set a precedent for his police officers.
"I used to do it personally, just to show to the guys that if I can do it, why can't you?" he said, according to a video posted by the Star.
"I'd go around in Davao with a motorcycle, with a big bike around, and I would just patrol the streets looking for trouble," he said. "I was really looking for a confrontation, so I could kill."
The president elaborated on the shootings Friday, calling them incidents with hostage-takers that saved people's lives.
But his comments already had prompted a warning from U.S. officials and handed critics ammunition to demand his ouster.
The United States on Thursday announced it had deferred a decision to send the Philippines a major economic aid package. White House spokesman Josh Earnest called his remarks "deeply troubling" and "certainly at odds with the Philippine government's stated commitment to due process and rule of law."
Sen. Leila de Lima, the president's most high-profile opponent, told CNN that his admissions amount to "mass murder" and grounds for impeachment.
Duterte, instigator of a bloody drug war during his presidency that has led to the death of more than 3,000 people, is known for his outlandish statements. He has compared himself to Hitler, declared he was not a killer and then threatened to kill officers who disobeyed him, and hinted at his own overuse of a powerful painkiller.
He has mentioned killing criminal suspects before, but his repeated remarks this week are the strongest declarations since he took office in June.
Duterte's popularity has only increased since then, and it's unclear what effect his comments will have throughout the island nation. Philippine officials who support the president have tried to play them down.
"The president always resorts to hyperbole," Justice Secretary Vitaliano Aguirre II said on Wednesday, according to the Star. "He always exaggerates just to put his message across."
Duterte's confirmation that he had killed challenged such claims.
"I would not have any second thoughts about cutting your head off," he told the paper on Friday, speaking about drug dealers. "Should I allow it to continue? And be like the Latin American states?"
Meyers is a special correspondent.