As the operation to retake the Iraqi city of Mosul gets underway, American troops are poised to expand their hands-on support to local forces battling the Islamic State. In a sign of the importance of the long-awaited offensive, military leaders are authorized to place U.S. forces advisers with Iraqi army battalions for the first time as they push toward militant lines, exposing U.S. forces to greater risks.

Military officials say the troops will remain away from the thick of combat and will limit their support to coordinating airstrikes and artillery fire, providing intelligence and helping plan troop movements. The U.S. role will not, they insist, look like the years following the 2003 invasion of Iraq, when U.S. troops who operated nominally in support of local forces often did the bulk of the fighting themselves. This time, U.S. advisers are “not meant to be the front-line troops,” Capt. Jeff Davis, a military spokesman, told reporters Tuesday.

Several days into the operation, Pentagon officials have provided only general information about where U.S. troops will be positioned and how exactly they will take part in the hoped-for advance. While American troops have been widely seen alongside local forces in forward positions east of Mosul, Pentagon press secretary Peter Cook on Monday said only that some forces were positioned “on the outskirts of the city.” He said it was not yet clear whether U.S. forces would be permitted to enter the city proper.

Officials’ reluctance to provide greater detail reflects a desire to protect U.S. advisers, attached to local units in small groups of about a dozen. It is also part of an effort to keep the U.S. role in the background, as Iraqi leaders have ruled out a combat role for foreign forces and local military commanders seek to claim victory after past defeats.

But questions about the extent of U.S. activities in the unfolding ground operation also serve to highlight the elasticity of the American advisory role, which has been employed in a host of counterterrorism conflicts since 2001, such as joint planning at heavily guarded headquarters and combat situations where U.S. troops fight alongside or even ahead of local forces.

David S. Maxwell, a former U.S. Army Special Forces colonel now at Georgetown University, said military advisers must grapple with the tension between nurturing local forces — who are often beset by deep, systemic problems — and accomplishing the overall mission.

“One of the challenges we saw in Iraq and Afghanistan was that there was so much pressure to accomplish the mission, that often U.S. partnered forces would take the lead,” he said.

Sometimes, haziness about the U.S. advisory role is rooted in confusion about the terms military officials use to describe these activities, including “advise, assist and accompany,” said William Wechsler, who until last year served as deputy assistant defense secretary for Special Operations and counterterrorism.

“For the average person, advising sounds like something that is done from pretty far away, from time to time, with pretty limited responsibility,” he said. “To the military, advising is something that you’re doing on a day-to-day basis, shoulder to shoulder with your partners, and their success becomes your success.”

Another element is that while operational rules are established in Washington, decisions that shape the reality of what U.S. troops do are made by personnel in the field. This could come into play in the Mosul operation when advisers, who in most cases are authorized to accompany local troops only up to where they can protect themselves, make decisions about where that point stands.

“While policy limits must be established up front, there has to be some degree of flexibility allowed to those in the field,” said Wechsler, who is now a senior fellow at the Center for American Progress. “At the end of the day it’s a call for the people at the tactical level to determine what the last terrain feature might be.”

Advisers must also respond to changing battlefield threats that alter the advisory role, sometimes with deadly results. Last year, a Delta Force soldier became the first combat casualty in the renewed U.S. campaign in Iraq while providing support to an Iraqi rescue operation. When the Kurdish fighters came under unexpected fire, U.S. forces tried to come to their aid, and the U.S. soldier was killed.

Unlike some past operations, U.S. advisers in Mosul, who now number a couple hundred, are expected to remain in a secondary role. That’s partly because of Iraqi wishes but also a strong U.S. desire to avoid casualties. It may also reflect confidence that Iraqi troops, with the help of U.S. advisers and air power, can get the job done.

Loveday Morris and Kareem Fahim in Iraq contributed to this report. Thomas Gibbons-Neff contributed from Washington.