"The mission really is to protect American personnel in Erbil and potentially even in and around Baghdad," Pentagon spokesman John Kirby said at an emergency press briefing.
"And this is an important point - to try to help Iraqi security forces as they go after this threat," Xinhua quoted Kirby as saying.
"This is a threat inside their country, to their people, to their sovereignty, and ultimately this is an Iraqi fight to fight."
US President Barack Obama on Thursday said he authorized airstrikes "if necessary" against Islamic State (IS) militants, who have advanced into the Kurdish region of Iraq and seized the country's largest dam.
Obama said he had directed US forces to conduct airstrikes on the militants if they threaten the American citizens and military personnel there.
The UN Security Council has condemned the attacks by the Islamic State and expressed its "deep outrage" at the displacement of hundreds of thousands of Iraqis in the wake of the attacks.
Tens of thousands of refugees have fled into the mountains, hoping to reach the Kurdish region in north Iraq, but were trapped because of militant activity between the mountain and the Kurdish area, and are running short on food and water.