BARTELLA, Iraq — In a significant escalation of the battle for Mosul, elite Iraqi special forces joined the fight Thursday, unleashing a pre-dawn assault on an Islamic State-held town east of the besieged city, and the U.S. military announced the first American combat death since the operation began.

U.S. officials said the American service member died Thursday from wounds sustained in a roadside bomb explosion north of Mosul. More than 100 U.S. special operations forces are embedded with Iraqi units in the offensive, and hundreds more are playing a support role in staging bases.

The American had been operating as an explosive ordnance disposal specialist in support of the Iraqi Kurdish force known as the peshmerga, the U.S. officials said, speaking on condition of anonymity because they were not authorized to publicly discuss details.

Roadside bombs and other improvised explosive devices pose a particular danger to advancing Iraqi forces and the U.S. advisers who are with them.

The Islamic State group, which has occupied Mosul for more than two years, has prepared extensive defenses in and around the city.

As they charged toward the town of Bartella, nine miles from Mosul’s outskirts, the Iraqi special forces faced another favored weapon in the IS arsenal: armored trucks packed with explosives and driven by suicide bombers. The militants’ signature battlefield tactic, the weapons offered a glimpse at what Iraqi forces can expect as they close in on the extremists’ biggest urban bastion.

The pre-dawn assault on Bartella was part of a multi-pronged operation on eastern approaches to Mosul, Iraq’s second-largest city. Attack helicopters strafed militant positions as they advanced amid a hail of gunfire.

The U.S.-trained special forces, officially known as the Counter Terrorism Service, are widely seen as Iraq’s most professional and least sectarian fighters, and have served as the shock troops in previous campaigns against IS. They are expected to lead the charge into Mosul.

IS militants unleashed at least nine suicide car and truck bombs against the advancing troops, eight of which were destroyed before reaching their targets, while the ninth struck an armored Humvee, Lt. Col Muntadhar al-Shimmari told The Associated Press.

He did not give a casualty figure, but another officer said five Iraqi soldiers were wounded. The officer spoke on condition of anonymity because he was not authorized to release information.

“After we break them in Bartella, everywhere else, they will crumble,” said Maj. Gen. Fadhil Barwari. He said IS had few defenses in the town, which was almost completely empty of civilians. “They just left some snipers and suicide car bombs,” he said.