The Pentagon plans to send about 600 additional troops to Iraq to help launch a long-awaited offensive to retake Mosul in coming weeks, the most ambitious operation yet in the two-year military campaign against Islamic State.
The escalation, which has been approved by the White House, suggests the challenges U.S.-backed Iraqi ground forces will face in assaulting a heavily defended major urban center that is Islamic State's self-declared capital in Iraq and the largest city under its control anywhere.
An Iraqi victory in Mosul would effectively end Islamic State's self-declared caliphate in Iraq. President Obama would like to see the militants ejected or defeated in Iraq before he leaves office in January.
The Pentagon has about 6,000 troops, mostly operating as advisors and trainers, in Iraq. U.S.-led coalition warplanes based outside Iraq have carried out thousands of airstrikes since mid-2014.
Most of the new U.S. troops will be deployed to Qayyarah, an Iraqi airbase known as Q-West about 40 miles south of Mosul that has become a key staging base for the planned assault. Some also will be deployed to the Al Asad base, which is further west in Anbar province, to help with logistics.
A small component of special-operations forces also be dispatched to help Iraqi commanders gather and analyze intelligence from the battlefield.
Lt. Gen. Stephen Townsend, commander of U.S. forces in Iraq, must give his approval for the advisors to accompany Iraqi troops at the battalion level, meaning they could operate closer to the front line. U.S. advisors thus far have been largely confined to Iraqi division headquarters.
Defense Secretary Ashton Carter told reporters traveling with him in Albuquerque on Wednesday that the Pentagon plans to send "approximately 600 troops." A senior defense official said 615 troops will be deployed.
Carter said they will help Iraqi security forces and Kurdish fighters to "isolate and collapse" Islamic State's control over Mosul and "to protect and expand Iraqi security forces gains elsewhere."
"The Iraqi security forces have the combat role and we're in the support role, but I need to make clear: American forces combating ISIL are in harm's way," he said, using an acronym for Islamic State.
Carter said the Pentagon does not know if the militants plan to fight street by street for Mosul, as many have feared, or will abandon the city before the assault force, as has happened in several battles recently.
"Nor do we know whether they will be able to carry out whatever plans they have," he added. It's unclear "whether their fighters will stick with them, whether they will have the morale to do that, [and] whether the populace of Mosul will tolerate their continued presence in the city."
Surveillance drones and spy satellites have seen fighters building defensive positions inside Mosul although the militants have sought to hide their activity with smoke from burning oil and tires. U.S. planes have also sought to undermine support for the militants, and warn of the coming assault, by dropping millions of propaganda leaflets on the city.
The offensive, first promised in early 2015, has been repeatedly postponed as Iraqi security forces focused on retraining and on pushing the militants from other cities and towns closer to Baghdad, the capital.
Iraqi President Haider Abadi said in a statement Wednesday that he had requested more U.S. troops after "consultation" with President Obama. The two leaders met on Sept. 19 on the sidelines of the United Nations General Assembly in New York.
Abadi said "the role of the trainers and advisors is not combat, but for training and consultation only.
"It is our troops who will liberate the land," he said.
Kurdish troops will coordinate with Iraqi and coalition forces in the attack. The autonomous Kurdish government in northern Iraq has agreed to receive refugees of all ethnic and religious groups. An estimated 1 million civilians are in the city.
In recent weeks, advancing Iraqi and Kurdish ground forces have retaken towns and cities around Mosul and have cut off major supply routes on nearly all sides of the city.
Iraqi ground forces, backed by U.S.-led coalition airstrikes, pushed the militants from Sharqat last week, raising the Iraqi flag over a government compound. The town lies on the west bank of the Tigris River, about 50 miles south of Mosul.
Gen. Joseph F. Dunford Jr., chairman of the Joint Chiefs of Staff, said last week that the Iraqis being trained by the U.S.-led coalition for the Mosul offensive will be ready in early October.
The U.S. combat role in Iraq has increased steadily since Obama first authorized troops and air strikes in mid-2014. Special operations forces have accompanied Iraqi and Kurdish troops on raids and combat operations. Hundreds of additional troops are sent every few months to bolster support to Iraqi forces.
The Pentagon began extensive retraining of Iraqi troops in 2014 after Islamic State guerrillas first stormed in from Syria and seized large parts of western and northern Iraq.
The training has focused on nine Iraqi army brigades and three brigades of Kurdish fighters, known as peshmerga.
A brigade in Iraq can range from fewer than 1,000 troops to more than 3,000.
Officially, the Pentagon has deployed 5,262 troops in Iraq. But special operations forces and temporary deployments boost the total to more than 6,000. Three U.S. service members have died thus far fighting against Islamic State in Iraq.
"The president regularly asks: 'Could we make more progress if we devote additional resources to that effort?'" White House spokesman Josh Earnest said Tuesday. "And when the answer to that question is yes, the president has worked very closely with his team to find those resources and to devote them to that effort in a timely fashion."
Islamic State has held Mosul, Iraq's second largest city, since June 2014 when Iraqi forces in and around the northern city dropped their U.S.-supplied weapons and fled.
Although the Islamic State has steadily lost ground in Iraq and Syria, the Sunni militants have carried out daily suicide bombings in Shiite neighborhoods in Baghdad to stoke sectarian strife as well as destabilize the central government.
11:25 a.m.: This article was updated with additional details and more quotes from Ashton Carter.