A resurgent trend of "might makes right" has settled over vulnerable waters in the South China Sea, the commander of the US Pacific fleet has warned.

Speaking in Canberra on Wednesday, Admiral Scott Swift did not name China directly in a prepared speech, but was critical of "some nations" and what he called "unprecedented examples of aggressive construction and militarisation" on disputed territory.

China has been sharply criticised in the past year after dredging sand to build artificial islands at what had previously been coral atolls.

Admiral Swift warned a climate of uncertainty had been created by "thousands of acres of reclaimed land with newly constructed barracks, deep-water ports, extended runways, high power radars, surface-to-air missiles and squadrons of naval aircraft".