Vietnam protested to China on May 8 and demanded that it remove any missiles. The Philippines is studying whether to make its own protest.

But the neighboring countries will probably make few changes in view of any missiles, said Jonathan Spangler, director of the South China Sea Think Tank in Taipei. He sees no threat to marine shipping and notes that China regularly passes coast guard ships throughout the sea.

The missiles would mark a “slow progression” in a militarization aimed at stopping any invasion of Chinese-held islets, he said.