The Philippine president says he asked the U.S. ambassador why America did not deploy an armada of warships to pressure China to stop constructing man-made islands that are now at the heart of regional concerns in the disputed South China Sea.

President Rodrigo Duterte said in a speech Wednesday that U.S. Ambassador Sung Kim was unable to reply to the question when they met this week in southern Davao city, where he had a separate meeting with the Chinese ambassador. While berating the U.S., Duterte did not criticize China's behavior in his speech.

Duterte, however, said he will invoke an international arbitration ruling that declared China has no historic title to the disputed waters if Beijing drills for oil or gas in a contested shoal.