MOSUL, Iraq - Radios crackle, alerting the Iraqi forces advancing through the first of Mosul’s labyrinthine souks that drones circulating overhead have spotted an Islamic State car bomb moving towards the frontline.

The incoming vehicle-borne improvised explosive device (VBIED) is of terrifying proportions: an armour-plated industrial bulldozer on tracks. Its shovel is fixed low to push through Iraqi army frontline defences of high sand berms and armoured vehicles.

An ERD soldier prepares to advance in Mosul in March 2017 (Tom Westcott/MEE) Reinforcement soldiers, who have been resting in abandoned shops, their shutters torn open by fighting, leap to their feet, wrapping lengths of machine-gun ammunition around their shoulders and loading rocket-propelled-grenade (RPG) launchers.

Soldiers from Iraq’s Rapid Response Division (ERD), supported by Federal Police units, have been fighting from their latest strategic gain - Mosul’s riverside indoor souk - since dawn. Gradually the forces have fanned out through a network of narrow roads leading into the adjacent old souk.

A RPG operative waits to move forward, amid reports of incoming IS car bombs (Tom Westcott/MEE)

But these reinforcement units - one of ERD’s secret weapons - have been resting in preparation for unexpected IS resistance or counter-attacks.

A voice from the radio updates soldiers that the bulldozer VBIED is stuck, trying to plough its way through a high sand berm near the souk.

On the Corniche, forces gather alongside military vehicles. A young soldier loads a rocket into an RPG-29, a freestanding, larger-calibre version of the shoulder-held weapon.

Troops standing nearby plug fingers into their ears as he fires it down the battered remains of Mosul’s Corniche, which once bustled with shoppers and tourists.

An Abrams tank trundles forward from somewhere near the Ashur Hotel. A few hundred metres behind the frontline, the government-owned former five-star luxury hotel has been so battered by air strikes falling nearby that it now resembles a building under construction.

The tank struggles to pass through a narrow strip of tarmac between a huge crater left by an air strike and several armoured personal carriers. After some manoeuvring, it rolls into position and starts belting out heavy fire.

All weapons are now firing towards the bulldozer. Several bullets aimed too low turn into friendly fire, bouncing off the Iraqi army’s own bulldozer

All weapons are now firing towards the bulldozer. Several bullets aimed too low turn into friendly fire, bouncing off the Iraqi army’s own bulldozer, which has been reinforcing one of the sand berms.

“It’s coming, it’s coming,” shouts an ERD commander urgently.

Despite being pounded by heavy weapons, the IS bulldozer has ploughed through the first sand berm and is now grinding forward towards the forces. Through the deafening gunfire, the single boom of a direct hit from a Kornet - an expensive Russian anti-tank guided missile that Iraqi operatives have become adept at using to target IS car bombs - can be heard.

The bulldozer is finally stopped in its tracks.

After confrontation, rest

This is the second bulldozer VBIED that Iraqi forces have faced in the last week as they advance deeper into Mosul towards the Old City, which now lies just a few hundred metres in front of them.

“We stopped this one by killing the driver, and were then able to deactivate the IEDs it was loaded with,” ERD Captain Ferras tells MEE.

ERD soldiers in Mosul's riverside indoor souk in March 2017 (MEE/Tom Westcott) He gestures to the bulldozer, rendered almost unrecognisable by bullet-proof metal sheets and a network of grills designed to deflect incoming fire.

“If it had detonated, it would have created a huge explosion.”

The sinister-looking vehicle now stands between Iraqi army mortar and Improvised Rocket-Assisted Munition (IRAM) positions.

At the entrance to the dusty indoor souk, an ERD commander pauses to tell soldiers the IS bulldozer has been stopped. The news is greeted with silence and a few nods. Relentless machine-gun fire, both incoming and outgoing, ricochets outside the indoor souk. Cheerful melodies from soldiers’ mobile phones ring out incongruously.

Someone reminisces about his youth spent in Mosul.

“There was a huge casino right there beside the river, with all these expensive restaurants and cafes,” he says, gesturing towards the Corniche.

“So many westerners used to stay in the Ashur Hotel and we would watch them walking around, so blonde and fair and tall. It was like something from the movies.”

Most of the men chain-smoke in grim silence. One young federal policeman sings a little song under his breath, tapping out an accompanying rhythm with his ornate ring on the head of an RPG.

Air alive with missiles and gunfire

An ERD captain shouts into the souk that several more potential incoming VBIEDs have been spotted, before leaping into a Humvee and veering past troops towards the frontline.

His is one of two vehicles that are driving into IS sniper-fire to reach a small group of ERD soldiers who radioed back for support. They advanced deep into the Old Souk but have become trapped inside a building by several hidden IS snipers who unexpectedly became active.

At the end of the battered indoor souk, smoke billows from a burning Humvee (Tom Westcott/MEE) There is confusion about the trajectories of the newly spotted VBIEDs and IS have started attacking the riverside frontline with a series of mortars and RPGs. They clearly have no shortage of munitions.

Suddenly neither side of the covered indoor souk, which has acted as a haven for much of the day, seems safe. On the city side, the last remaining ERD soldiers dash up steps and out into the Old Souk, stepping over the bodies of several IS fighters felled during the morning’s battle.

On the Corniche, soldiers surge forward into battle, leaving a handful of Federal Police waiting under the awning of the souk.

The air is alive with missiles and gunfire. IS mortars rain down around the indoor souk, rattling the battered shutters. All the glass has already been shattered but remnants tinkle down into the debris-strewn corridors. A poorly aimed IS mortar round lands in the middle of the River Tigris, sending an extraordinary waterspout of clear water high into the air.

Intense sniper-fire whizzes down narrow streets

It’s clear that the IS bulldozer was the opening gambit of what has now become a fierce counter-attack as a barrage of RPGs starts pounding the frontline. The Iraqi military have faced unexpectedly modest resistance along the Corniche, advancing after coalition and Iraqi warplanes laid waste to riverside buildings occupied by IS. But the solid grouping of armoured vehicles which moved into frontline positions has suddenly become vulnerable.

“Ali, Ali. Turn your vehicle around,” yells a commander to one of the Humvees at the front, near the sand berm. The driver cannot hear above the gunfire. When he eventually does, he reverses into another vehicle.

As the Humvee’s body catches fire, the piles of machine-gun ammunition inside start exploding

An IS RPG hits a Humvee’s engine with a huge explosion. It bursts into flames. A soldier runs over to yank open the heavy doors, pulling out its occupants who, reeling with dizziness from the impact of the hit, manage to stagger towards the relative safety of the indoor souk. Smoke pours from the engine. The commander yells for someone to put out the fire but no one dares move towards it and into what has become a dangerous line of fire.

A random RPG hurtles into the chaos of the frontline from the opposite riverbank, from east Mosul, which was liberated almost two months ago, but where Iraq counter-terrorism forces still regularly arrest IS sleeper-cells. It does not explode. More RPGs fired from IS positions inside and along the edges of the Old Souk land around the frontline.

A federal policeman and an ERD soldier warm themselves by a fire in a riverside souk (Tom Westcott/MEE) As the Humvee’s body catches fire, the piles of machine-gun ammunition inside start exploding. A flying bullet hits a soldier in the face. He staggers away from the billowing cloud of black smoke with blood pouring from his face.

In the chaotic wake of the explosion, intense IS sniper-fire starts whizzing down one formerly quiet narrow street of the Old Souk, from the direction of a mosque’s minaret. The IS sniper, it seems, had been biding his time. And this is a road all the soldiers must cross to get back to the indoor souk.

The beginning of the battle

Today’s fierce IS counter-attack causes only modest losses - one Humvee and a handful of minor injuries - but it temporarily halts the progress of advancing ground forces.

With soldiers having gained some 400 metres of hard-won new territory, the advance has already been a success.

Mosul's former five-star Ashour Hotel now resembles a construction site (MEE/Tom Westcott)

This is the beginning of the much-anticipated battle for the Old City of west Mosul, where its narrow, winding streets will restrict the use of armoured vehicles and most advances will be made on foot.

As rain-clouds roll in over the River Tigris, troops start reinforcing their frontline positions in preparation for the night ahead.

This article is available in French on Middle East Eye French edition.