The amphibious transport ship USS Mesa Verde with 550 Marines on board entered the Gulf on Monday to support possible US 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 US defence official said.
President Barack Obama has said he is considering military action, short of putting US troops in Iraq, to aid Baghdad's efforts to stop the insurgency.
The USS Mesa Verde joins the aircraft carrier USS George HW 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.
Separately, Saudi Arabia said on Monday it rejected foreign interference in Iraq, after Islamist militants seized broad swathes of the country.
In a cabinet statement published on the official news agency SPA, Riyadh blamed the unfolding crisis on years of "sectarian and exclusionary" policies.
It also urged the "quick formation of a national consensus government."