The USS Stout, a guided missile destroyer, is "in the Mediterranean, heading and moving east" to relieve the Mahan, said the official, who said both ships might remain in place for the time being.
Other destroyers in the region -- the Ramage, the Barry and the Gravely -- criss-cross the Mediterranean and could launch their Tomahawk missiles toward Syria if so directed by US President Barack Obama.
The defense official, speaking on condition of anonymity, did not say how long the Mahan would stay in the area before returning to its home port of Norfolk, Virginia, which it left in December 2012.
The US Navy keeps as a closely guarded secret the number of Tomahawk missiles that each ship carries but it is estimated to be 45.
The US defense official also indicated that the aircraft carrier USS Nimitz and its escorts remain in the area of the US Fifth Fleet, which extends from the Red Sea to the Gulf and Arabian Sea.
Defense Secretary Chuck Hagel, currently on a week-long trip to Southeast Asia, told the BBC that US forces were in place and "ready to go" if Obama gives the order but no such decision has yet been made.