"It should be clear that ISIL is not only a threat to the stability of Iraq, but a threat to the entire region," State Department spokeswoman Jen Psaki said.
She stressed that Washington backed "a strong coordinated response to push back against this aggression" from the Islamic State of Iraq and Levant (ISIL).
The militants launched a major assault on Iraqi security forces late yesterday and grabbed Iraq's northern province of Nineveh, including its capital Mosul.
ISIL was drawing "strength from the situation in Syria, from which it transfers recruits, sophisticated munitions, and resources to the fight in Iraq," Psaki said in a statement.
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The most powerful militant group in Iraq, ISIL is a key force in the rebellion against President Bashar al-Assad in neighbouring Syria.
US officials in Washington and Baghdad were "tracking events closely in coordination" with the Iraqi government, Psaki said.
A top US diplomat, deputy assistant secretary Brett McGurk, has been in Iraq since the weekend for talks on how to combat the ISIL threat.
"We will continue to work closely with Iraqi political and security leaders on a holistic approach to diminish ISIL's capacity and ability to operate within Iraq's borders.