HANOI (Reuters) -U.S. aircraft carrier USS Nimitz left the South China Sea on Monday morning heading west, according to data from ship tracking website Marine Traffic, after a reception for its planned port call in central Vietnam was cancelled. One of the sources said the U.S. Embassy in Hanoi had informed him about the cancellation, due to "an emergent operational requirement". The Nimitz Carrier Strike Group conducted maritime security operations in the South China Sea last week, as "part of the U.S. Navy's routine presence in the Indo-Pacific," according to the website of the Commander of the U.S. Pacific Fleet.