The commander of the Maritime Self-Defense Force has warned that all of the South China Sea could be "covered by China's sphere of military influence" if Beijing uses artificial islands it has built there for military purposes.

In a speech Wednesday at a think tank in Washington, Adm. Tomohisa Takei underlined the importance of ensuring that the South China Sea remains "free and open waters" so that the Indian-Pacific area is an "Ocean of Prosperity."

Should the freedom of navigation be threatened, Takei said that "an unexpected incident at sea can occur as a result."