美防长将视察游弋南中国海的美国航母

U.S. Secretary of Defense Ash Carter is visiting a U.S. warship in the South China Sea to emphasize the United States' commitment to security in that area, where territorial disputes have place China at odds with several of its neighbors.

Carter announced the visit Friday in Manila at the closing ceremonies of the U.S.-Philippines joint military exercises.

He said security in the region is like oxygen -- if there is enough of it, it goes unnoticed. But, he added, "When you don't have enough, you can think of nothing else."

Carter said activities like the joint military exercises the United States and Philippines just concluded are important to protecting security in the region. Carter said the first South China Sea joint patrol was carried out in March. Defense officials say a second joint patrol was completed in early April.

China protested Friday. Beijing said the United States is taking a "lopsided approach" favoring China's rivals for territory in the South China Sea. China claims almost the entire South China Sea, and has aggressively carried out island-building and patrols in waters that are claimed by other countries.