Philippine President Rodrigo Duterte has sought to allay fears about China's construction of military bases on man-made islands in the South China Sea, saying these were built to defend against America rather than confront neighboring states.

On Monday, Duterte also blamed past governments for not building up the country's defense in the Spratly archipelago at a time when Beijing was only starting to build its artificial islands and turning them into military bases.

"We did nothing," the firebrand leader said in a speech to Chinese-Filipino businessmen. On China's construction, he added: "It's not intended for us. The contending ideological powers of the world or the geopolitics has greatly changed. It's really intended against those who the Chinese think would destroy them and that is America."