As a walkie-talkie carried word of another casualty from an Islamic State mortar attack, an Iraqi policeman peered through leaves at enemy positions just across the Tigris River. He kept his head low to avoid snipers but also had an eye on the sky.

Minutes later, the militants sent a drone overhead. It carried out surveillance and dropped an explosive. Then mortar bombs landed nearby, sending the policemen running for safer ground.

More than three months into the battle to drive them from their biggest stronghold, the hardline Sunni militants of Islamic State remain lethal and determined, despite being driven from the eastern half of the city of more than a million people.

Few are more acutely aware of the danger they pose than police Lt-Colonel Falah Hammad Hindi, who instructed his men to take cover as mortars landed ever closer.

"The weapon of choice is the drone," said Hindi, whose unit faces sometimes 16 drone attacks in a single day as well as mortar bombs and snipers.

His unit, charged with holding ground while Iraqi troops prepare to expand their offensive to west Mosul, is stationed on a former Islamic State training ground and closed military area on the east bank of the Tigris.

He has gained insight into the militants' thinking and strengths and gave a frank assessment of their capabilities, starting with the snipers he can spot without binoculars.

"The snipers are highly effective. They are foreign fighters, the most committed," Hindi told Reuters.

When Islamic State swept into Mosul in 2014 and declared a caliphate on land straddling Iraq and Syria, they attracted volunteers from as far afield as Afghanistan and Tunisia and also won many sympathizers in Mosul, Iraq's second largest city.

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Mosul's predominantly Sunni population was angered by Iraq's Shi'ite-dominated army, accusing it of widespread abuses of their minority sect, allegations rejected by the government.

Islamic State exploited that resentment, hunting down and executing members of the army and police as it tightened its grip on Mosul and simultaneously attracting local volunteers who saw it, initially, as a bulwark against Shi'ite power.

New recruits were trained at the site where Hindi and his men are now based, a former plant nursery, family park and state-owned honey farm.

Here they learned the group's credo, a version of Islam even more radical than its predecessor in Iraq, al Qaeda.

Trees and lush greenery provided ideal cover from air strikes, so jihadists could become indoctrinated in relative safety. To be extra cautious, the militants built an underground tunnel with sandbags for air raids.

Aside from weapons training, jihadists learned discipline. They were made to suffer in the cold when it rained or snowed.

"Some men were fed only a few potatoes per week," said Hindi, who lost a brother to an Islamic State attack. "Others were only allowed to eat three dates per day. They became battle-ready here."

In order to battle Islamic State militants positioned about 500 meters across the river at a hospital and hotel, policemen study their training for clues.

They also rely on intelligence from residents of west Mosul, turned against Islamic State by the brutality of its rule.

"They hide in their homes and provide information about the jihadists. Their movements, their weapons," said Hindi, 32.

The risks are high. Some informers have been executed.

The campaign for west Mosul will likely involve far tougher and more complex street fighting because the west's narrow streets mean far fewer tanks and armored vehicles can be deployed against Islamic State.

The militants are also expected to put up a much fiercer fight in the western half of Mosul because the battle will determine whether their self-proclaimed caliphate will survive.

"They have no escape route in the west so they will fight to the death," said Hindi.

The conflict will play to the group's strengths: suicide bombers, whom Hindi said were being reserved and positioned for that battle, car bombs and booby traps.

Just as Hindi and his men made it to what they thought was a more secure area, they took cover behind trees, after concluding another drone was circling above. A mortar bomb landed a few hundred meters away.

Eventually he sat in his office, discussing future challenges over cups of sweet tea. Another senior officer, who also lost a brother to jihadists, paid a visit.

"Two days ago, 38 terrorists snuck over the river in a boat to carry out an attack," he told Hindi. The men were killed.

"They want to show they are still a threat and in control."

Even if Islamic State is defeated in all of Mosul, the Shi'ite-led government and army faces the daunting task of easing sectarian tensions and winning over the Sunni city, once a vibrant trade hub.

"It all depends on how the army behaves," said Hindi. "If there are abuses again, a new generation of Daesh (Islamic State) fighters will be back."