Trump offered to mediate on South China Sea issue but Duterte slapped him in the face.

http://www.khmertimeskh.com/5090214/trump-offers-mediate-south-china-sea/

Duterte rejected Trump's offer outright because he knew the ulterior motive of Trump.  Trump is not really genuine in mediating the South China Sea issue and he just wants to stir more troubles in South China Sea for USA political interest.

Duterte saw through Trump's ruse immediately because the USA would create troubles wherever it was involved and the Middle East was a superb example.