Washington diplomatic officials are seeking an explanation after Philippines President Rodrigo Duterte said his country would "separate" from long-time allies, the United States.
Speaking at the start of an official visit to China, Mr Duterte said the Philippines will now look to Beijing for help in what would be a major foreign policy shift.
He told a business forum in Beijing that America had lost him.
"Your honours, in this venue I announce my separation from the United States, both the military but economics also."
He also told the audience there were three countries against the world - the Philippines, China and Russia.
State Department spokesperson John Kirby said it was not clear what Mr Duterte meant and they were asking for clarification.
"It is inexplicably at odds with the very close relationship we have with the Filipino people, as well as the government there, on many different levels - not just from a security perspective."
With relations souring further, Assistant Secretary of State Daniel Russel was due to visit Manila this weekend on what the State Department said was a previously scheduled trip.
He would seek to clarify Mr Duterte's comments, the State Department said.
US attempts to raise questions about Mr Duterte's campaign against drugs, in which more than 3000 people have been killed since he took office in June, have previously drawn angry denunciations by Mr Duterte.
He has derided US President Barack Obama as a "a son of bitch" and said he should "go to hell".
If the US chose to respond more vigourously, it could decide to cut military aid to the Philippines or make it contingent upon an end to the drug killings or more careful judicial procedures.
But Philippines officials have suggested their country could live without the US assistance, and overtures to China and Russia suggest they might seek assistance elsewhere.
Mr Duterte's trade secretary, Ramon Lopez, said $US13.5 billion in deals would be signed during Duterte's China trip, though it was unclear how much of that amount was in the form of final deals rather than preliminary agreements.
The White House said current US direct investment to the Philippines was over $US4.7 billion.
Earlier this week there were protests outside the American embassy in Manila, calling on the US to end its military presence in the Philippines.
Mr Duterte's latest outburst, less than three weeks before the US presidential election, casts further doubt on the seven-decade US-Philippine alliance and threatens to further undermine Mr Obama's faltering "pivot" to Asia as a counterbalance to China's growing assertiveness.
One US official, who did not want to be identified, said there had been an active internal debate in recent months on how far to go in criticizing Duterte's government on human rights and that the measured tone adopted was not as strong as some aides would have liked.
"There is no question that Duterte is ... trying to play the well-worn game of playing us off against the Chinese," another US official said, on condition of anonymity.
- BBC / Reuters