By David Alexander

WASHINGTON (Reuters) - The U.S. military conducted freedom of navigation operations last year challenging maritime claims by 19 countries, from China to Argentina, asserting U.S. transit rights in defiance of efforts to impose restrictions, the Pentagon said on Wednesday.

It was the largest number of countries challenged in more than a decade, establishing the program's return to levels from before the wars in Iraq and Afghanistan, when U.S. forces had to curtail operations because of other priorities, officials said.

The rise in operations was due in part to an increased focus on Latin America, where the U.S. military challenged the claims of half a dozen countries, including Argentina, Brazil and Venezuela, according to the Pentagon's annual Freedom of Navigation Report for 2014.

The military has regularly conducted operations disputing some of China's maritime claims in recent years and did so again in 2014. A U.S. official, speaking on condition of anonymity, said none of those claims were related to Beijing's dredging to create islands out of reefs in the South China Sea.

The official said China has not attempted to assert maritime claims around those newly created islands and would not be able to do so under international law.

"Under international law an island is a naturally formed feature, and it specifically says in international law that an artificial island is not entitled to a territorial sea," the official said. Any effort to create a maritime zone around those former reefs would be "a legal impossibility," he said.

The United States carries out freedom of navigation operations by sending Navy ships and military aircraft into maritime areas that nearby countries have tried to restrict in some way. The operations aim to show that the international community has not accepted the restrictions.

Iran and the Philippines have been the most frequently challenged countries over the years, mainly because they sit astride heavily traveled sea lanes whose use they have tried to limit or govern.

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Tehran has tried to restrict passage through the Strait of Hormuz to signatories of the Law of the Sea Convention, which the United States has not endorsed. Manila has designated the Sulu Sea and other areas as archipelagic waters over which it has complete sovereignty, a claim Washington disputes.

Freedom of navigation operations, which began in 1979, are coordinated by the State and Defense departments and are meant to be consistent with the United Nations Convention on the Law of the Sea, even though Washington has not adopted it.

(Reporting by David Alexander; editing by Gunna Dickson and Leslie Adler)