The amphibious transport ship USS Mesa Verde with 550 Marines on board entered the Gulf on Monday to support possible U.S. action to help Iraq's Shi'ite-led government combat a Sunni Islamist insurgency that has overtaken large areas of the country's north, a U.S. defense official said.
President Barack Obama has said he is considering military action, short of putting U.S. troops in Iraq, to aid Baghdad's efforts to stop the insurgency.
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The USS Mesa Verde joins the aircraft carrier USS George H.W. Bush, which moved into the Gulf on Saturday along with the guided-missile cruiser USS Philippine Sea and the guided-missile destroyer USS Truxtun.
"USS Mesa Verde is capable of conducting a variety of quick reaction and crisis response operations. The ship carries a complement of MV-22 Osprey tilt-rotor aircraft," the Pentagon said in a statement.
CNN first reported the USS Mesa Verde's movement into the Gulf.