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.

The carrier had planned to visit Danang City later this week, but two sources, including one diplomat, said a formal reception slated for June 20 had been called off.

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.

Data from Marine Traffic showed the carrier on Monday morning was moving west in the direction of the Middle East, where the battle between Israel and Iran is escalating.

Later, two U.S. officials told Reuters that the U.S. military has moved a large number of refueling aircraft to Europe to provide options to President Donald Trump as Middle East tensions soar. 

The officials also said the U.S. aircraft carrier Nimitz was heading to the Middle East, although one official said the movement was pre-planned.