U.S. 2013 freedom of navigation operations targeted China, and even the Philippines

U.S.operations challenging Iranwere aimed at rejecting Tehran's effort to restrict the Strait of Hormuz to ships from nations that have signed the U.N. Convention on the Law of the Sea, an accord the United States has not formally adopted.

The operations, which began in 1979, are coordinated by the State and Defense departments and are meant to be consistent with the U.N. Law of the Sea Convention, even though Washingtonhas not formally adopted the agreement.

U.S.operations in the 2013 fiscal year also challenged claims by Cambodia, India, Indonesia, Libya, Malaysia, Maldives, Oman, Taiwan and Vietnam. All countries but Cambodiawere targeted more than once.

Since 1991, the United Stateshas conducted more than 300 freedom of navigation operations challenging maritime claims by 53 different countries worldwide, from Albania, Ecuadorand Denmark to Pakistan and Yemen.

Iran and the Philippineshave been challenged most frequently. Iranhas appeared on 19 of the 21 lists submitted to Congress since 1991, while the Philippines has appeared on 18. Cambodia, the Maldives, India and Oman also frequently appear.

China has been on the list 11 times, the same as Indonesiaand one less than Burma.

The most frequent U.S. complaint is with countries that measure the start of their territorial waters by drawing a straight line between two points on the coast or along offshore islands, thereby enclosing a vast expanse of sea.

The United States targets about a dozen countries per year for challenge, with the high ranging to 27 countries in 1998 and dropping into the low single digits at the height of the U.S. war in Iraq.