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The US-led coalition appears confident that fighters of the so-called Islamic State (IS) will be defeated in Mosul. But the battle for Iraq's second largest city has already been going on for six months and the Iraqi forces have only just reached the edges of the old city.

Optimism has been tempered by the slow progress of what has become a brutal fight for every street. In the words of the US coalition spokesman Col John Dorrian "the fight's been very, very slow and very, very hard… its gut-busting".

We joined the Iraqi forces about to launch yet another assault to take more territory. Over the past few weeks, the initial advance has slowed to a crawl with the front lines relatively static. They want to break the deadlock. This is the story of just one battle.

They mass under the cover of darkness. The same Iraq units who've been fighting here for months. The troops both battle hardened and battle weary. It's supposed to be a surprise dawn attack. But IS will be lying in wait. As we move forward on foot we soon come under fire.

We follow one of the Iraqi commanders, Maj Mohammed, as he sets up a makeshift headquarters in an abandoned house that's already seen heavy fighting. There's IS graffiti on the walls.

As his troops advance there is a sudden, panicked call on the radio. It's his first casualties. They've walked into a booby-trapped building: several men have been injured and they're calling for help.

There's no let-up in the fighting as dawn breaks amid the heavy thud of machine guns firing on both sides.

We hear coalition aircraft overhead. Then a whoosh and a thud, followed by an explosion. One of the IS heavy machine guns has been silenced by a coalition airstrike.

There are several more over the next few hours - uncomfortably close. An Iraqi soldier smiles and points as a bomb travels at speed towards another IS-held building nearby.

In the distance we can now see the black flag of IS flying. And nearer, the buildings and the holes in the wall from where they're firing.

There's another whoosh, thud and boom and then a plume of smoke from an air strike. We're told to stay inside because the Iraqi forces have heard a small IS drone. They're often armed with grenades.

Five hours later, the battle is still raging over the same few streets. An Iraqi armoured bulldozer tries to clear a path through the wrecked cars and rubble to help the advance. But the driver is targeted by an IS rocket-propelled grenade. There's a frantic effort to free him from the cabin. His comrades eventually succeed but he's lost limbs and is bleeding profusely.

No-one can question the bravery of the Iraqi forces, but you can see the losses and the expectations of victory weighing heavily on their shoulders.

We ask to leave when IS begin to mortar the Iraqi positions. The impact sends brick and concrete flying through the air. The building we are taking shelter in shudders and then there's a cloud of debris. Someone shouts "Gas!" but thankfully it's not.

We leave in the same Humvee we first arrived in. The seats are now stained in blood from ferrying the wounded. By the end of the day, the Iraqi forces have taken a few more streets.

But this is unforgiving, urban warfare and for the Iraqi forces there is still a mountain to climb.