By Karen Lema MANILA (Reuters) - The United States is open to the possibility of joint naval patrols with the Philippines in the South China Sea, a U.S. diplomat said on Wednesday, stressing it would continue to exercise "freedom of navigation" in the disputed waters. China claims most of the South China Sea, through which more than $5 trillion of world trade is shipped every year. Vietnam, Malaysia, Brunei, the Philippines and Taiwan have rival claims.