CAMP SWIFT, Iraq — The assault on Mosul, Iraq’s second largest city, is bringing American forces into their most significant role in Iraq in years, in terms of numbers and presence on the front lines.
The lead-up to the assault has already brought some US forces into combat with the militants. Special forces carry out raids alongside Iraqi troops inside Islamic State-held territory around Mosul.
As Iraqi forces prepare for the operation to retake the city, those raids have increased in frequency, said a coalition official, who spoke on condition of anonymity as he was not authorized to brief the media.
The United States has also sent Apache helicopters to aid in the Mosul fight, the Pentagon said, a step that was not taken when Iraqi forces retook the western cities of Ramadi and Fallujah.
The number of US troops in Iraq has steadily grown over the past two years to now nearly 6,000 service members, up from almost none after the 2011 withdrawal from Iraq.
The latest group, numbering nearly 600, began to deploy in September to Qayara air base, the facility 30 miles south of Mosul that is to be the main staging ground for the assault on the city. Trucks have been rolling in the base for weeks with supplies and equipment, preparing it so coalition warplanes will be able to operate there.
‘‘You’ve got to look at Mosul as the crown jewel right now,’’ said Major General Gary Volesky, the head of US ground forces in Iraq, regarding the build-up of forces. The deployments have ‘‘all been targeted to assist in the Mosul attack.’’
Besides the hundreds of special forces, most of the American personnel operate back from the front lines, coordinating coalition airstrikes, tracking Iraqi ground troops, sharing intelligence, and helping plan operations.
Thirteen years ago, Chase Snow’s father was among the American troops who moved into the Iraqi city of Mosul during the US-led invasion that toppled Saddam Hussein. Now Snow, a US Army specialist, is deployed in Iraq to help in the fight to retake the city from the Islamic State.
Snow, from Nashville, is with the 101st Airborne Division, advising Iraqi officers carrying out the Mosul operation. His father was also with the 101st in Mosul in 2003.
On his Iraq deployment, Snow carries the same American flag his father kept with him on all of his tours and his father’s good luck charm: a St. Michael prayer card.
‘‘I know my father never thought I would be coming to Iraq,’’ Snow said.
US presence at bases closer to Mosul in the lead up to operation is ‘‘essential’’ to the advise-and-assist mission, said US Army Colonel Brett Sylvia, the commanding officer at Camp Swift, a small coalition base outside Makhmour, some 45 miles southeast of Mosul.
‘‘If you’re not there, then you don’t have a voice,’’ Sylvia said, standing in front of the bank of televisions and desktop monitors that he says constitutes the forward edge of the battle for his men.
As of last week, there were 4,565 US troops in Iraq, the Pentagon said. That doesn’t include1,500 more troops considered there ‘‘on temporary duty,’’ whose number changes daily, the US officials said.
US troop levels in Iraq peaked at 157,800 during the 2008 surge under then-President George W. Bush, the Pentagon said. More than 140,000 US troops were in Iraq when President Obama took office in 2009.
Obama drew down the forces until the complete withdrawal of late 2011 removed all combat troops from the country, leaving behind only a few hundred US trainers, mainly civilians, to assist Iraqi security forces.
US forces began returning after the Islamic State overran Mosul in the summer of 2014 and blitzed across much of northern, central, and western Iraq, joining it to territory it holds in Syria.
Over the past year, three American service members have been killed by the Islamic State in Iraq, revealing the increasingly active role of US forces in a fight the Pentagon initially refused to describe as combat.
In October 2015, Master Sergeant Joshua Wheeler was killed as he and dozens of other US special forces participated in a raid alongside Iraqi Kurdish forces to free Islamic State-held prisoners.
Months later, Marine Staff Sergeant Louis Cardin was killed when Islamic State fighters attacked a fire base near Camp Swift.
By the time of the third American death — Navy SEAL Charles Keating, who was killed in May — Defense Secretary Ashton Carter immediately described it as a combat death. ‘‘He was in a firefight and he died in combat,’’ he said.