Isis may be outnumbered and out-gunned in Mosul, but their snipers are lethally efficient weapons in the fierce defence of their last stronghold in Iraq. A strategically positioned, well hidden, sniper can pin down the advancing Iraqi forces for days. The deadlock is sometimes broken only when the sniper makes a mistake and is taken out by an equally well hidden and patient marksman on the Iraqi forces' side.

Iraqi snipers take up position, surrounded by spent bullet casings and foam mattresses as they wait for their opposite numbers to show themselves. "Just like we're hiding from them, they're hiding from us," said al-Moqdadi al-Saeedi, commander of a sniper unit that is part of the Iraqi Rapid Response division.

"Eighty-five to 90 percent of it is about hiding," said Saeedi. "Hiding and endurance: they complement each other. Some people can hide but they can't endure: that person cannot be a sniper."

Mosul snipers
A sniper of the Iraqi rapid response forces aims his rifle during a battle with Isis militants in Mosul Thaier Al-Sudani/Reuters

Caffeine pills help them stay alert while they wait. Taif Tala, one of the unit's members, told Reuters: "Sometimes we don't eat, but you forget your hunger." He said they try to figure out where the snipers are hiding. "We imagine ourselves to be the enemy – if I were the enemy, where would I hide?"

As snipers themselves, the Iraqi unit understand the Isis sniper capability better than most. "The sniper's role is more defensive than offensive," said Saeedi. Isis marksmanship is not to be underestimated. Saeedi showed Reuters a picture on his mobile phone of the scope of his rifle, shot straight through by an jihadist sniper in a previous battle. "They say snipers are cowardly fighters because they are always behind the troops and they hide," said Abo Ali, a member of the unit. But he added: "We hide, and we kill."

As Iraqi troops move ahead, the sniper unit advances behind them, choosing a position from which to target the enemy without being spotted themselves. Sometimes it proves impossible to deal with an enemy sniper, so the unit calls in heavier weapons or an air strike.

An elderly local man told Reuters how Isis fighters had used the top floor of his home to fire sniper rifles and machine guns at advancing Iraqi forces, while he and his family cowered downstairs. If the militants had held out a few minutes longer, an air raid would have brought the building down on top of them. "Thank God, they retreated from here and troops arrived to find just us. One of the soldiers told us they were five minutes away from calling in an air strike because of the resistance coming from our house," he said.

The fighters, some of whom spoke Russian, left their posts in the upstairs bedrooms and retreated through the next-door house via holes they had knocked through the walls, he said.

Iraqi forces say they aim to dislodge Isis from west Mosul within a month. "Despite the tough fighting... we are moving ahead in persistence to finish the battle for the western side within a month," Lieutenant General Talib Shaghati told Reuters.

Isis tunnels Mosul
A man cries over his son who he said was killed by an Isis sniper Thaier Al-Sudani/Reuters

The few thousand militants still fighting in west Mosul are overwhelmingly outnumbered by a 100,000-strong array of Iraqi forces. There is little doubt Iraqi forces will eventually prevail over the militants, who are both outnumbered and overpowered, but even if it loses Mosul, Islamic State is expected to revert to their insurgent tactics of old.