The US says it is sending a warship into the Gulf to provide President Obama with military options should the situation in Iraq deteriorate further.
US Defence Secretary Chuck Hagel ordered the USS George HW Bush, which carries dozens of fighter jets, to be moved from the North Arabian Sea.
Meanwhile, Iran says it could be prepared to work with the US to fight Sunni insurgents in Iraq.
The insurgents have seized several cities and are closing in on Baghdad.
Fighting under the banner of The Islamic State in Iraq and the Levant (ISIS), they regard Iraq's Shia majority as "infidels".
Iraqi security forces, bolstered by an increasing number of Shia and Kurdish militiamen, are trying to take a stand in Salahaddin and Diyala provinces, to the north of the capital.
The aircraft carrier will be accompanied by the guided-missile cruiser USS Philippine Sea and the guided-missile destroyer USS Truxtun. They were due to arrive in the Gulf late on Saturday.
US Secretary of State John Kerry has called on Iraq's political leaders to put their differences aside to defeat the Islamist offensive.
Kerry urged the government to ratify recent election results without delay and stick to a constitutional time-frame to form a new government.
ANALYSIS: RICHARD GALPIN, BBC NEWS
A long line of pick-up trucks and cars sped through the streets of central Baghdad on Saturday filled with heavily armed men; a cacophony of blaring horns, sirens and pumping music heralding their arrival.
It was evidence that the call to arms made on Friday by the most senior Shia religious leader Grand Ayatollah Ali al-Sistani, was being heeded in the capital as in other cities, particularly in the Shiite-dominated south.
The convoy was manned by members of a Shiite militia called the Mahdi Army, which has sprung back to life in recent days, after lying low following its notorious role in the sectarian conflict with the minority Sunni community, which came to an end about six years ago.
One cleric, sitting with a driver and bodyguard in an expensive four-by-four vehicle, said they were prepared to fight to the death and "splash their blood" to rid Iraq of the ISIS "terrorists".
Other members of the militia said they would help defend Baghdad and head further north to bolster the regular army on the frontlines in Diyala and Salahaddin provinces.