The first of up to 300 US military advisers began their mission in Baghdad Tuesday to help the Iraqi army, but the Pentagon said the American troops were not taking on a combat role.
The primary task of the advisers was to evaluate the state of the Iraqi forces and not to turn the tide against militants from the Islamic State of Iraq and the Levant (ISIL), which have swept across western and northern Iraq, the Pentagon's press secretary said.
"This isn't about rushing to the rescue," Rear Admiral John Kirby told reporters.
"These teams will assess the cohesiveness and readiness of Iraqi security forces ...and examine the most effective and efficient way to introduce follow-on advisers," Kirby said.
The US troops, which included special operations forces, would relay their findings to commanders within "the next two to three weeks."
He did not say how long the advisers would be in place but said: "This is a limited, short-term duration mission."
Two teams of about 40 troops, which were drawn from the US embassy in Baghdad, "have started their new mission," Kirby said.
An additional 90 troops have arrived in Iraq to set up a joint operations center in the Iraqi capital and another 50 are due to deploy in the next few days, he said.
Combined with troops already stationed at the US embassy and others sent to bolster security there, the American military's presence in Iraq was now at about 500 forces, officials said.
After the stunning onslaught of ISIL militants, President Barack Obama announced plans to send the advisers to Baghdad last week while leaving open the possibility of eventual air strikes against the extremists.
The US military, which has deployed an aircraft carrier group to the Gulf, was ready to carry out bombing raids if called upon, Kirby said.
"We remain postured to do that," Kirby said.
But for the moment, the focus was on looking at the Iraqi forces and examining how additional teams of American advisers should be organized, he said.
"This is just the first day of the establishment of these assessment teams."
After swiftly advancing across a swath of territory in the north and west, ISIL forces are trying "to solidify those gains and to continue to threaten Baghdad," Kirby said.
He also said the United States had expanded its surveillance flights over Iraq, with manned and unmanned aircraft, and now was conducting 30 to 35 sorties a day.
In recent days, Iraqi forces have fended off assaults by ISIL at the Baiji oil refinery in the north, the country's largest, and the strategic western town of Haditha.