Duterte, whose bloody war on crime had already drawn international condemnation, sparked fresh outrage on Friday when he likened his deadly crime war with Hitler's efforts to exterminate Jews.
Duterte said he was merely reacting to critics who drew comparisons between him and the Nazi leader.
"So I said, 'sure I am Hitler, but the ones I will kill are these (drug addicts)'," Duterte said in a speech broadcast on national television.
"It was never my intention but the problem was I was criticised using Hitler, comparing to me. But I was very emphatic. I will kill the three million."
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Duterte, 71, won elections in May in a landslide after a campaign dominated by his pledge to eradicate drugs in society by killing tens of thousands of people.
Since taking office on June 30, police have killed more than 1,200 people and about 1,800 others have died in unexplained circumstances, according to official figures.
Duterte threatened to cancel a defence accord with the United States, the Enhanced Defense Cooperation Agreement (EDCA), that went into force in January.
That agreement, sealed under the administration of Duterte's predecessor, Benigno Aquino, allows US forces access to five Philippine bases to help counterbalance a growing Chinese presence in the South China Sea.
"This EDCA is an official document... But it does not bear the signature of the president of the republic," Duterte said.
The United States and the Philippines are longtime allies bound by a mutual defence pact.
However Duterte has repeatedly threatened to move away from the United States and forge closer ties China and Russia.
Duterte last month branded US President Barack Obama a "son of a whore".