The Philippines is tilting away from its traditional ally the United States towards China in a bid to "normalise" relations following a longstanding territorial dispute, the country's incoming ambassador to Beijing said Monday.

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."

Beijing claims most of the South China Sea despite competing claims from the Philippines and other Asian countries, but a UN-backed tribunal ruled in July that China's claims had no legal basis in a resounding victory for Manila.

Duterte's decision to set aside the territorial conflict in exchange for Chinese investment and aid has given Beijing a boost in its quest for more control over the strategically vital waters.

The incoming envoy, a former Beijing-based journalist, said Manila was open to working with China to access resources in the disputed region.

"The Chinese viewed the Philippines as a geopolitical pawn or Trojan horse of the US. Now they look at us as a friendly neighbour."

He added that relations with the US plunged after Washington criticised Duterte's crackdown on crime.

"The problem came after they began lecturing him. The president considers it an internal affair," he said.

"The Chinese don't comment on your internal affairs."