President Rodrigo Duterte of the Philippines said on Friday that he had stabbed someone to death when he was a teenager, the latest claim from a leader who has boasted of personally killing people while defending a violent crackdown on drugs.

“When I was a teenager, I had been in and out of jail, rumble here and there,” Mr. Duterte said during a speech to Filipinos in Danang, Vietnam. “At the age of 16, I already killed someone.”

He said he had stabbed the person “just over a look,” and he asked his audience in Vietnam to imagine what he could do as president.

“I won’t let you off the hook” if you mess around with Filipinos, he said. “Never mind about the human rights advocates.”