As Washington steps up its activities near man-made islands built by Beijing in disputed South China Sea waters, some experts are saying that Japan, with its newly empowered military, could play a larger role in the increasingly fraught quarrel.

The United States, which is reportedly weighing "freedom of navigation" patrols within 12 nautical miles of the artificial islands, has pointed to nearby shipping lanes that it says are vital to the free flow of trade.

Media reports earlier this month said the U.S. had decided to conduct the patrols, which Washington uses to challenge territorial claims in oceans and airspace it considers excessive.