US Defense Department official said the USS Stethem, a guided-missile destroyer, sailed within 12 nautical miles of Triton Island's territorial water boundary, part of the Paracel Islands in the South China Sea, in an operation meant to challenge the competing claims of all three nations. It was the second of what the United States calls a freedom of navigation operation conducted under the administration of President Donald Trump.<br/> <br/>US President Donald Trump is due to speak to his Chinese counterpart Xi Jinping in a pre-arranged call today. In May, another US naval ship sailed within 12 nautical miles of an artificial island built by China in the South China Sea. The incident happened just before leaders of the two countries were due to speak on the phone.<br/> <br/>Beijing has called the presence of the warship a serious political and military provocation and said it would use all necessary means to defend national sovereignty and security. <br/><br/>China's Ministry of Foreign Affairs said in a statement that the U.S. ship had made an unauthorized entry into China's territorial waters. It said China had sent battleships and fighter jets to warn off the Stethem. China has been embroiled in maritime disputes with several of its regional neighbours in recent years.