The Pentagon spokesman, Rear Admiral John Kirby, says Secretary of Defense Chuck Hagel gave the order that the USS George H.W. Bush move from the North Arabian Sea to the Gulf Saturday.
Kirby says the order provides President Barack Obama "additional flexibility should military options be required to protect American lives, citizens and interests in Iraq."
A guided-missile cruiser and a guided-missile destroyer will accompany the aircraft carrier.
Meanwhile, the French news agency (AFP) reports an Iraqi air strike hit a convoy of Kurdish forces in eastern Iraq, possibly by mistake, killing six fighters. And mortar fire in central Iraq hit a recruitment center for civilians volunteering to fight the Islamist militants, killing six people. Neither report has been confirmed.
Hundreds of Iraqi young men flocked to volunteer centers across Baghdad and elsewhere starting Saturday to join the fight against Islamist militants who have advanced across the north this week.
The volunteers are responding to a call to arms from Iraq's most revered Shi'ite cleric, Ayatollah Ali al-Sistani.
A spokesman for the ayatollah Abdul Mahdi al-Karbalaie urged Iraqis to "fight the terrorists in defense of their country, its people and its holy sites." He said fighting the militants is "everyone's responsibility."
Iraqi officials say government forces have regained some territory with the help of volunteers.
Iraqi Prime Minister Nouri al-Maliki has told army officers in Samarra that volunteers are arriving there to help soldiers defeat the militants.
Maliki vowed again Saturday to defeat Sunni militants who have captured large chunks of territory north and west of the capital, Baghdad.
The prime minister also said troops who abandoned their positions and left their uniforms in the streets in the northern city of Mosul earlier this week must return to their units or face possible severe punishment, including the death penalty.
The Iraqi Army, meanwhile, claimed that government forces have regained their momentum, after a string of recent defeats.
Fighting continued Saturday in scattered regions north of Baghdad, with conflicting reports over which side — forces loyal to Maliki's government, or a mix of Sunni Islamist militant groups — had the upper hand.
Iran offers help
In another development Saturday, Iranian President Hassan Rouhani said his country is ready to help Iraq if asked, and would consider working with Tehran's longtime foe, the United States, in fighting Sunni extremists if Washington decides to take strong action against the fighters. Iran has developed close ties in recent years with the Shi'ite-led government in Baghdad.
U.S. President Barack Obama said Friday his national security advisers are preparing "a range of options" for U.S. assistance to Iraq's government as it faces an assault by al-Qaida inspired Islamist militants.
The president said the militants who have overrun parts of Iraq are a threat to the Baghdad government and people throughout the country, and pose an active threat to American interests as well. He said division among Iraq's leadership has led to the current crisis.
According to news reports by Reuters, U.S. Secretary of State John Kerry said the Washington's assistance would only work if Iraqi leaders "overcame deep divisions." The top U.S. diplomat communicated the information in a Saturday phone call with Iraqi Foreign Minister Hoshyar Zebari.
In quick strikes this week, militants of the Islamic State of Iraq and the Levant, or ISIL, took control of Mosul, Iraq's second largest city, and advanced within 90 kilometers of Baghdad.
Late Thursday, ISIL fighters seized the towns of Jalawla and Saadiyah in the ethnically divided eastern province of Diyala.
A spokesman for the Sunni militants vowed they would push into Baghdad and on to Karbala, a city southwest of Baghdad that is one of the holiest sites for Shi'ite Muslims.
Some information for this report comes from AP and Reuters.