The U.S. suffered its first combat death in the Mosul offensive Thursday when a service member died of wounds from the detonation of a roadside bomb.

The service member, who was not identified, "died of wounds sustained in an improvised explosive device blast" in northern Iraq, according to a press release from Combined Joint Task Force-Operation Inherent Resolve.

The release did not give a location for the incident but a U.S. official, speaking on grounds of anonymity, told USA Today that the service member was involved in supporting local forces in the drive to retake the Islamic State of Iraq and Syria stronghold in northwestern Mosul.

More than 100 U.S. advisers and Joint Terminal Attack Controllers to guide airstrikes have moved forward with the attacking forces in a support role, along with AH-64 Apache helicopters, according to the Defense Department. There are currently about 5,000 U.S. troops serving in Iraq, primarily to advise and assist Iraqi forces.

The announcement of the death came a day after Army Maj. Gen. Gary Volesky, commander of Combined Joint Forces Land Component Command-Operation Inherent Resolve and the 101st Airborne Division (Air Assault), said that U.S. advisers accompanying friendly forces were operating well behind the forward line of troops.

The death was the fourth in combat for U.S. troops since they deployed to Iraq in 2014 to support the Iraqi forces against ISIS.

In 2015, Army Master Sgt. Joshua Wheeler, 39, was killed during a raid with Kurdish forces on an ISIS prison compound. Last March, Marine Staff Sgt. Louis Cardin, 27, died in an ISIS attack on a firebase about 60 miles southeast of Mosul. In May, Navy SEAL, Special Warfare Operator 1st Class Charles Keating IV, 31, was killed in a firefight after his quick reaction force came to aid an advisory team.

The death of the service member in the Mosul offensive came a day after a U.S. military member was killed and another was injured in an insider attack in Kabul, Afghanistan. A U.S. civilian was also killed and two more American civilians were also wounded in the incident at a base in the capital of Afghanistan.

The first U.S. casualty in the Mosul offensive was suffered on the fourth day of the advance in which Iraq's elite Counter-Terror Services units joined the fight and armored columns reportedly drove to within four miles of the city. Kurdish Peshmerga forces attacking from the east also opened a new front to the north of Mosul.

ISIS sought to impede the advance through mostly abandoned desert villages with sniper, mortar and rocket fire, improvised explosive devices and VBIEDs (Vehicle-Borne Improvised Explosive Devices).

The CTS forces, which have received the most extensive training and best equipment from the U.S., led the successful assaults that retook the flashpoint towns of Ramadi and Fallujah in Anbar province. They were also expected to lead the assault into Mosul.

U.S. and coalition aircraft took on targets of opportunity as the advance continued. CJTF-OIR said in a statement Thursday that three airstrikes near Mosul engaged two ISIS tactical units and two staging areas, destroying four vehicles, four ISIL-held buildings, four fighting positions, three containers, two mortar systems, a tunnel entrance, a bunker, an artillery system, a front-end loader, a weapons cache, a heavy machine gun and a tanker trailer.

Iraqi Prime Minister Haider al-Abadi said that progress thus far was the result of a new unity of purpose and coordination between the Iraqi Security Forces and the Kurdish fighters. On Wednesday, Volesky, said he had never seen such cooperation between the Kurds and the Iraqis in his five tours in Iraq.

In a video briefing to an international meeting in Paris on the future of Mosul, Abadi said "The forces are pushing towards the town more quickly than we thought and more quickly than we had programmed in our campaign plan." The Kurdish and Iraqi forces were "fighting harmoniously together," he said.

As the advance continued, and fighting opened up on new fronts, there was growing concern in the U.S.-led coalition on the fate of the hundreds of thousands of civilians trapped in Mosul who have been living under harsh ISIS rule since June 2014.

Mosul was Iraq's second-largest city with a population of about two million before the ISIS takeover. About one million are estimated to remain, according to the United Nations.

Abadi said he had instructed "the military on all the fronts to open corridors to citizens and IDPs (Internally Displaced People), to all the people who want to flee the fighting." Leaflets dropped over Mosul by Iraqi and coalition aircraft have also warned residents to seal their homes and to stay away from ISIS positions.

Although Abadi praised the current cooperation among the attacking forces, there were still concerns in the coalition over the roles that will be played by the various factions once the battle for the city itself begins.

Several reports said that the Shia Popular Mobilization Forces, which have links to Iran's Revolutionary Guards, were moving towards the mostly-Sunni city behind the main columns.

In a briefing to the Pentagon on Wednesday, Volesky said the coalition had been assured by Abadi that the PMF units would not enter the city.

In the taking of Fallujah earlier this year, the PMF units remained on the outskirts where they allegedly committed atrocities on fleeing civilians while “vetting" them to make sure they were not ISIS fighters, according to human right groups.

The advancing forces also had yet to deal with the problem of the Turkish military presence on the outskirts of the town of Bashiqa to the north of Mosul. In increasingly testy exchanges, Abadi has called for the withdrawal of the Turks from Iraqi territory but his demands have been rejected by Turkish President Recep Tayyip Erdogan.

The small Turkish military contingent outside Bashiqa has been training tribal fighters and Erdogan has demanded that they be included in the Mosul operation. Turkey has also been pressing for the coalition to allow its warplanes to join in airstrikes against ISIS in Iraq.

Defense Secretary Ashton Carter was to meet Friday in Turkey with his Turkish counterpart, Defense Minister Fikri Isik, on the anti-ISIS campaign on the first stop of a week-long swing that will also take him to the United Arab Emirates and NATO ministerial meetings in Paris and Brussels.

At a Pentagon news conference Thursday, Carter stressed that Turkey was a NATO ally and a member of the anti-ISIS coalition but “anybody who operates in Iraq needs to do so with respect to Iraqi sovereignty. We work through issues as they arise. I expect that will continue."

-- Brendan McGarry can be reached at brendan.mcgarry@military.com

-- Richard Sisk can be reached at Richard.Sisk@Military.com.