Manila has been one of Washington's most loyal allies in Asia, but Philippine President Rodrigo Duterte has threatened to end the decades-long alliance after the US criticised his bloody war on drugs that has killed over 5,000 people since he took office in June.

His fiery rhetoric against the US has been followed by overtures to China as he has sought to assuage Beijing's concerns over Manila's competing claims to the South China Sea.

The new Philippine ambassador to China, Jose "Chito" Sta. Romana, told AFP the move represented "a strategic shift in our foreign policy".

"We were one-sidedly imbalanced in favour of the US," he said.

"We are not abandoning our alliance with the US.... We are basically trying to normalise our relations with China."