U.S. troops serving as advisers were moving with elite Iraqi units that have advanced into the eastern sector of Mosul but were remaining behind the "forward line of troops," a military spokesman said Wednesday.

Air Force Col. John Dorrian, a spokesman for Combined Joint Task Force-Operation Inherent Resolve, would neither confirm nor deny that the U.S. advisers were now inside the northwestern Iraqi city.

He said the troops were accompanying Iraqi Counter Terror Service units who were experiencing "extremely tough fighting" in advancing about a third of the way into the eastern sector against Islamic State fighters.

"We're not going to verify the location of U.S. forces inside Mosul," Dorrian said in a video briefing from Baghdad to the Pentagon. "We do remain behind the forward line of troops.

"We do advise the CTS … we'll go where they need us," he added. "Our advisers are close by" and "they will most certainly defend themselves" if need be.

President Barack Obama has barred U.S. troops from ground combat in Iraq and Syria while authorizing them to serve as trainers and advisers, which has frequently put them in combat situations.

Earlier this year, Army Lt. Gen. Sean MacFarland, then commander of CJTF-OIR, was authorized to place U.S. advisers with the Iraqis at the battalion level and that authority remains with his successor, Army Lt. Gen. Stephen Townsend.

In addition to the advisers, U.S. JTACs, or Joint Terminal Attack Controllers, have also been moving forward with Iraqi Security Forces and Kurdish Peshmerga forces in the campaign to oust the Islamic State of Iraq and Syria from Mosul, the last ISIS stronghold in Iraq.

U.S. forces have suffered one combat death since Oct. 17, when the Mosul offensive began as the ISF moving mostly from the south and the Kurdish Peshmerga forces from the east began pressing towards the outskirts of the city that once was home to two million residents.

On Oct. 20, Chief Petty Officer Jason Finan, 34, of Anaheim, Calif., who was attached to a SEAL team advising the CTS, died of wounds suffered from an improvised explosive device. Finan was assigned to the Navy’s Explosive Ordnance Disposal Mobile Unit 3 based in Coronado, Calif.

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, a 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.

In his briefing, Dorrian said that the heavy resistance in Mosul put up by ISIS included the use of "Mad Max"-type vehicles that ISIS fighters had rigged with makeshift armor. The vehicles were used by suicide drivers to get close to the Iraqi forces before detonating explosives carried in the back of the vehicle.

The Peshmerga recently stopped one of the Mad Max VBIEDs, or Vehicle Borne Improvised Explosive Devices, with .50 caliber machinegun fire and were now using it to train troops on coping with the threat, Dorrian said. He said that about 60 ISIS VBIEDs had been destroyed since the Mosul offensive began.

While main ISF units press the fight in Mosul, Iranian-backed Shia militias known as Popular Mobilizations Units, which are part of the ISF, were moving closer to the town of Tal Afar to the west of Mosul in the effort to cut off a supply and escape route for ISIS, Dorrian said.

Dorrian would not give a timeline for the taking of Tal Afar but said it was his understanding that the PMUs would not enter the town itself and would leave that task to the main ISF units.

Despite the slow progress in Mosul, Dorrian said that "eventually the enemy is going to break. Ultimately, the Iraqis are going to take that city" and ISIS will then "be back to just being a garden variety terrorist group" in Iraq.

Dorrian echoed similar remarks earlier this week by Joint Chiefs Chairman Gen. Joseph Dunford following a brief visit to Iraq. Dunford said that in his judgment the taking of Mosul will reduce ISIS "back to an insurgency with terrorist actions and get them to a level where Iraqi Security Forces with a minimum level of outside support will be able to manage the violence inside Iraq."

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