US troops have been deployed to Iraq to assist the Iraqi army in combating a growing Sunni militant insurgency, the Pentagon has announced.
Nearly half the 300 special operations soldiers promised by President Barack Obama are in Baghdad or on the front lines of the fight.
The rest are expected within days.
Meanwhile, US Secretary of State John Kerry called for regional unity to expel the Sunni ISIS rebels who have taken large swathes of Iraq.
On Tuesday, two teams totaling 40 US troops began work assessing Iraqi troops on the front line, the Pentagon said.
An additional 90 personnel will work in Baghdad to set up a new joint operations command centre.
Those teams will be joined by an additional four teams of 50 troops each in the next few days.
The Obama administration has stressed the troops are not intended as operational forces but instead are there to advise the Iraqis and provide intelligence.
The Iraqi government had requested American air strikes, but Barack Obama has been reluctant to do anything that could lead to accusations the US was taking sides in a sectarian conflict.
The insurgents, spearheaded by Islamists fighting under the banner of the Islamic State of Iraq and the Levant (ISIS), have overrun much of north and west Iraq, including the second-biggest city, Mosul.
The violence has claimed at least 1,075 lives in Iraq in June alone, most of them civilians, a UN human rights team has reported.
The UN said the figures, which include a number of verified summary executions, should be viewed as an absolute minimum.