The Pentagon is considering sending U.S. military aircraft and ships to assert freedom of navigation around rapidly growing Chinese-made artificial islands in the disputed South China Sea, a U.S. official said Tuesday.

Defense Secretary Ash Carter requested options that include sending U.S. military ships and aircraft within 12 nautical miles (22 km) of reefs that China has been building up in the disputed Spratly Islands, the official said.

Such a move would directly challenge Chinese efforts to expand its influence in the disputed region by literally adding territory through a massive island-building exercise.