Media playback is unsupported on your device Media caption The BBC had exclusive access to an Iraqi mission near Mosul

The Iraqi army has been stepping up air strikes on the Islamic State (IS) group's positions in northern Iraq in preparation for an expected offensive to retake the city of Mosul.

BBC Persian's Nafiseh Kohnavard is the first ever journalist to be allowed to fly on an air mission over the occupied city.

It's 01:00 at Erbil Airport in northern Iraq and army mechanics are carrying out final checks on a military helicopter. It's a Russian-made Mi-17 modified to carry two rockets.

As we arrive, soldiers are scrawling messages like "This one is for you Islamic State!" and "You are doomed!" on them.

The missions are a joint operation between the Iraqis and the Americans, and most of the pilots are US-trained.

We're joined by General Ahmed Thwenee, an air force veteran who explains that helicopters can deliver more precise strikes because they fly at low altitudes. Of course, that leaves them more exposed to fire from the ground, as the general experienced first-hand when he was shot in the leg on a previous mission.

As our crew pose for photos on the tarmac, a US military advisor asks if we're sure we want to go. "This is going to be a dangerous operation," he says.

Image caption Nafiseh Kohnavard and her team aboard the helicopter

Bombing his home

The target is a sulphur factory 25 miles (40km) outside Mosul. We're told IS are using it to produce bombs and to train suicide bombers. We fly with two other helicopters and we're escorted by a US war-plane high overhead.

It's freezing cold inside the chopper. A gunner is sitting at the open doors, machine-gun at the ready.

Down below in the darkness I see the twinkling lights of a string of villages captured by Islamic State in the past year.

Hassan our pilot is from Mosul. He still has relatives in the city and heard recently that his house had been occupied by IS.

It's clear from his expression just how personal these missions are for him. He tells us that he's asked for permission to bomb his house, but was told it's too big a risk to the civilian population.

Image caption The Iraqi pilots celebrate a successful mission

A burst of flame

After half an hour we reach the target and hover overhead, waiting for orders to strike. Although it's incredibly noisy on board, a scary silence seems to descend.

Then suddenly it's all go. The first two helicopters dive down and dance over the target, firing their rockets.

Then it's our turn to swoop in.

We see a burst of flame right below us, and then after a couple of seconds we hear a big boom. Hassan swoops the helicopter round in a big arc to make sure they've hit their target. The machine gunner gives the thumbs up and we're off.

The whole thing took just twelve minutes but it felt like the longest twelve minutes of my life.