China has protested American surveillance flights over the islands in the South China Sea where it is filling in reefs & shoals, and constructing military airfields, and warned the United States about entering the twelve-mile limit around those reefs.
The United Nations Convention on the Law of the Sea (UNCLOS) only gives nations maritime rights over actual islands, not filled-in territory, and the islands must be able to support human habitation, and economic activity. China's posturing is therefore not supported by law.
Additionally, the Peoples' Republic is relying upon the so-called nine-dash line in support of its claims. Trouble is, those territorial claims were made, in the 1940s, by the Republic of China, also known as Taiwan, and never assigned or transferred to the Peoples' Republic.
The problem is the US intends to press the issue, on the grounds of freedom of navigation, and the Philippines and Vietnam will suffer increases in their Country Risk if there are any incidents, due to their proximity.