China says it is seriously concerned about reports saying the U.S. is considering sending military ships and planes to challenge Chinese claims to South China Sea islands.

Foreign Ministry spokeswoman Hua Chunying said Wednesday the U.S. needs to clarify its stance on the matter. She said China's upholds freedom of navigation in the area but that doesn't mean foreign forces can operate freely there.

The Wall Street Journal quoted U.S. defense officials as saying Defense Secretary Ash Carter had asked for ideas about how to address China's moves to reinforce islands it occupies in the strategically vital area.

While the U.S. military already operates in the South China Sea, crossing the 12 nautical mile (22.2 kilometer) territorial limit around the islands could raise tensions if China chose to respond.