Bangkok: Philippines President Rodrigo Duterte insulted and mocked Americans during a strongly anti-US speech in China's Great Hall of the People where he announced his "separation" from the United States.
"Americans are loud, sometimes rowdy. Their larynx is not adjusted to civility," Mr Duterte said before mimicking an American accent.
"They are discourteous people," he said, adding that is wise not to do business with them because "that is the surest way of losing your money".
Video of the speech aired on Rappler.com shows those in the audience laughing at the jibes.
The US State Department said it would seek an explanation from the Philippines, calling the remarks baffling and at odds with the 70-year US-Philippines alliance that has helped underpin stability in Asia.
"We are going to be seeking an explanation of exactly what the president meant when he talked about separation from the US," said State Department spokesman John Kirby.
Daniel Russel, the US's top diplomat for East Asian and Pacific Affairs, is scheduled to visit Manila this weekend.
Asked about Mr Duterte's remarks, Foreign Minister Julie Bishop said: "The Australian government believes the United States has played a constructive role in maintaining peace and stability in our region and beyond. While Australia has benefited enormously from our close relationship with the United States, it is the right of each sovereign nation to determine its international relationships in their national interest."
In remarks that will alarm policy-makers in the US and its allies in Asia and the Pacific, Mr Duterte said: "In this venue, your honours in this venue, I announce separation from the United States.
"Both in military, not maybe social, but economics also. American has lost," he said.
"I've re-aligned myself in your ideological flow and maybe I will also go to Russia and talk to [President Vladimir] Putin and tell him that there are three of us against the world – China, Philippines and Russia. It's the only way."
Earlier he said it was "time to say goodbye" to the United States as he steered his country towards Beijing's sphere of influence during a red-carpet state visit to China.
Since being swept into power at May elections pledging to wipe out illegal drugs and crime within months, 71-year-old Mr Duterte has launched tirades against the US, including referring to Barack Obama as the "son of a whore" after the US expressed concern about reports of extra-judicial killings across the country.
Policy-makers from Washington to Tokyo to Canberra are trying to understand why the long-time mayor of the southern city of Davao has taken such a fiercely anti-American line in his first months as president of a country which has been the US's closest ally in Asia for decades.
Even some of his staunchest supporters at home are worried about his abrupt abandonment of the US, which complicates Mr Obama's foreign policy "pivot" to Asia at a time of heightened concern over China's aggressive claim to the strategic waters of the South China Sea.
Mr Duterte told Filipinos in Beijing in another speech last Wednesday that he was once denied a US visa when he planned to visit a girlfriend in college.
He has said previously he was molested by an American priest when he was a child.
The president has repeatedly referred to the US, the Philippines' former colonial power, as having bullied their "little brown brothers". He frequently mentions how Ferdinand Magellan came to the Philippines in 1521 to begin the subjugation of Filipinos.
Mr Duterte often refers to an incident in 2002 when the US embassy in Manila allegedly helped an American escape responsibility for a mysterious explosion in a Davao hotel room. He was mayor of the city at the time.
His four-day visit to China with 400 businesspeople has brought the Philippines firmly into China's sphere of influence.
Philippines Trade Secretary Ramon Lopez said $US13.5 billion ($17.6 billion) in deals were signed during the trip.
Chinese President Xi Jinping described his meeting with Mr Duterte as a "milestone" in ties.
He said China and the Philippines were brothers and they could "appropriately handle disputes", though he did not mention the South China Sea in front of reporters.