Duterte's tirades against the former colonial power are routine during his speeches and he said on Wednesday he once believed in Washington, but had since lost respect for the Philippines' biggest ally.
In Washington, U.S. State Department spokesman John Kirby said he regarded Duterte's latest salvo as "inexplicably at odds with the close relationship that we continue to have with not just the Filipino people, but the Filipino government."
The State Department halted the sale of the assault rifles to the Philippine police after staff from U.S. Senator Ben Cardin's office said he would oppose it, Senate aides told Reuters on Monday.
Aides said Cardin, the top Democrat on the U.S. Senate Foreign Relations Committee, was reluctant for the United States to provide the weapons given concern about human rights violations in the Philippines during Duterte's bloody, four-month-old war on drugs.
"Look at these monkeys, the 26,000 firearms we wanted to buy, they don't want to sell," Duterte said during a televised speech. "Son of a bitch, we have many homemade guns here. These American fools."
More than 2,300 people have been killed in police operations or by suspected vigilantes as part of Duterte's anti-narcotics effort, which was the linchpin of his election campaign.