Iraqi forces have launched the push for Mosul's militant-held western half just over four months after the operation to retake the city from Isis officially began.

The fight for Iraq's second largest city — roughly split by the Tigris River into an eastern and a western section — has seen periods of swift territorial gains as well as weeks of gruelling urban combat with high civilian and military casualties. Iraqi and US-led coalition officials say they expect the fight for the west to be more difficult as it is denser, with narrower streets and home to more civilians.

In the days after Iraqi forces announced the start of the Mosul operation, Iraq's special forces quickly retook a handful of largely empty villages along the city's east that brought them to Mosul's edge in nearly November. Once inside the city the tempo of operations changed dramatically with barrages of car bombs inflicting heavy military and civilian casualties.

In pictures: Mosul offensive Show all 40 1 /40 In pictures: Mosul offensive In pictures: Mosul offensive A doctor carries an Iraqi newborn baby at a hospital in Mosul, Iraq July 18, 2017. Reuters In pictures: Mosul offensive Iraqi girls play at a yard of a school in Mosul, Iraq July 18, 2017alal Reuters In pictures: Mosul offensive A woman on crutches who is a relative of men accused of being Islamic State militants is seen at a camp in Bartella, east of Mosul, Iraq July 15, 2017. Picture taken July 15, 2017. Reuters In pictures: Mosul offensive A displaced girl, who fled from home carries a doll at Hamam al-Alil camp south of Mosul, Iraq July 13, 2017. Reuters In pictures: Mosul offensive Iraqi federal police members and civilians celebrate in the Old City of Mosul on 9 July 2017 after the government's announcement of the "liberation" of the embattled city. Iraqi Prime Minister Haider al-Abadi's office said he was in "liberated" Mosul to congratulate "the heroic fighters and the Iraqi people on the achievement of the major victory" AFP/Getty Images In pictures: Mosul offensive A picture taken on 9 July 2017, shows a general view of the destruction in Mosul's Old City. Iraq will announce imminently a final victory in the nearly nine-month offensive to retake Mosul from jihadists, a US general said Saturday, as celebrations broke out among police forces in the city. AFP In pictures: Mosul offensive Members of the Iraqi federal police raise the victory gesture as they ride on a humvee while advancing through the Old City of Mosul on 28 June 2017, as the offensive continues to retake the last district held by Islamic State (IS) group fighters. AFP/Getty Images In pictures: Mosul offensive Smoke billows as Iraqi forces advance through the Old City of Mosul on 26 June 2017, during the ongoing offensive to retake the last district held by the Islamic State (IS) group. AFP/Getty Images In pictures: Mosul offensive An Iraqi man wearing the green scarf of the Shi'ite faith kisses an Iraqi Army soldier on safely reaching the Iraqi forces position as Iraqi civilians flee the Old City of west Mosul where heavy fighting continues on 23 June 2017. Iraqi forces continue to encounter stiff resistance with improvised explosive devices, car bombs, heavy mortar fire and snipers hampering their advance. Getty Images In pictures: Mosul offensive A picture taken from the inside of an Iraqi forces armoured vehicle shows residents walking through a damaged street as troops advance towards Mosul's Old City on 18 June 2017, during the ongoing offensive to retake the last district still held by the Islamic State (IS) group. Military commanders told AFP the assault had begun at dawn after overnight air strikes by the US-led coalition backing Iraqi forces. They said the jihadists were putting up fierce resistance. AFP/Getty Images In pictures: Mosul offensive Iraqi Army soldiers advance in a destroyed street after an Iraqi forces airstrike targeted an Islamic State sniper position 17 June 2017 in al-Shifa, the last district of west Mosul under Islamic State control. IS snipers, as well as car and suicide bomb attacks continue to hinder the Iraqi forces efforts to retake the final district. A series of airstrikes by Iraqi helicopter gunships attempted to hit multiple Islamic State sniper positions in al-Shifa. Getty Images In pictures: Mosul offensive An Iraqi soldier frisks a displaced Iraqi man at a temporary camp in the compound of the closed Nineveh International Hotel in Mosul on 16 June 2017 which was recovered by Iraqi troops from Islamic State group fighters earlier in the year. A screening centre set up in the compound's fairgrounds sees a constant stream of Iraqis fleeing the battle for Mosul, awaiting their turn to be checked by the Iraqi forces who are searching for suspected Islamic State (IS) group members. The small fairground lies at the end of a pontoon bridge across the Tigris recently opened to civilians that is the only physical link between the two banks of the river. AFP/Getty Images In pictures: Mosul offensive Iraqis staying at the al-Khazir camp swim in a river near the camp for internally displaced people, located between Arbil and Mosul on 11 June 2017. AFP/Getty Images In pictures: Mosul offensive Iraqi government forces drive on a road leading to Tal Afar on 9 June 2017, during ongoing battles to retake the city from Islamic State (IS) group fighters. AFP/Getty Images In pictures: Mosul offensive An Iraqi policeman carries a poster bearing an image of Mosul's iconic leaning minaret, known as the "Hadba" (Hunchback), on 22 June 2017. AFP/Getty Images In pictures: Mosul offensive Iraqis stand in line to receive food aid in western Mosul's Zanjili neighbourhood on 7 June 2017, during ongoing battles as Iraqi forces try to retake the city from Islamic State (IS) group fighters. Living conditions in Mosul have again deteriorated since the start of the Iraqi government's offensive on the city in October in which they retook a large part of the west of the city. AFP/Getty Images In pictures: Mosul offensive Displaced Iraqis carry lightbulbs and sacks as they evacuate from western Mosul's Zanjili neighbourhood as government forces advance in the area during their ongoing battle against Islamic State (IS) group fighters on 13 May 2017 AFP/Getty Images In pictures: Mosul offensive A member of the Iraqi Counter-Terrorism Service (CTS) flashes the victory gesture as he patrols in western Mosul's al-Islah al-Zaraye neighbourhood on 13 May 2017 AFP/Getty Images In pictures: Mosul offensive Iraqi army soldiers from the 9th armoured division on a truck flash the sign of victory as they drive back from Mosul to the town of Qaraqosh (also known as Hamdaniya) Getty In pictures: Mosul offensive Members of Iraqi forces flash the sign of victory on their vehicle as they advance towards Hammam al-Alil area south of Mosul Getty In pictures: Mosul offensive A member of Iraqi security forces gestures in Hammam al-Alil, south of Mosul, Iraq Reuters In pictures: Mosul offensive Iraqi children, one flashing the sign of victory, greet Iraqi army's soldiers from the 9th armoured division in the area of Ali Rash, adjacent to the eastern Al-Intissar neighbourhood of Mosul Getty In pictures: Mosul offensive Peshmerga forces look at a tunnel used by Islamic State militants near the town of Bashiqa, east of Mosul, during an operation to attack Islamic State militants in Mosul, Iraq Reuters In pictures: Mosul offensive An Iraqi soldier takes a photograph with his phone as his comrade stands next to a detained man, whom the Iraqi army soldiers accused of being an Islamic State fighter, who was fleeing with his family in the Intisar disrict of eastern Mosul, Iraq Reuters In pictures: Mosul offensive Iranian Kurdish female members of the Freedom Party of Kurdistan (PAK) hold a position in an area near the town of Bashiqa, some 25 kilometres north east of Mosul Getty In pictures: Mosul offensive Iraqi families, who fled their homes in Hamam al-Alil, gather on the outskirts of their town Getty In pictures: Mosul offensive Displaced people walk past a checkpoint near Qayara, south of Mosul, Iraq AP In pictures: Mosul offensive Iraqi families who were displaced by the ongoing operation by Iraqi forces against jihadists of the Islamic State group to retake the city of Mosul, are seen gathering in an area near Qayyarah In pictures: Mosul offensive A boy who just fled Abu Jarbuah village is seen with his family at a Kurdish Peshmerga position between two front lines near Bashiqa, east of Mosul, Iraq Reuters In pictures: Mosul offensive An Iraqi child eats a pomegranate upon the arrival of Iraqi forces in the village of Umm Mahahir, south of Mosul Getty In pictures: Mosul offensive People who just fled Abu Jarbuah village sit as they eat at a Kurdish Peshmerga position between two front lines near Bashiqa, east of Mosul, Iraq Reuters In pictures: Mosul offensive A couple who just fled Abu Jarbuah village are escorted by Kurdish Peshmerga soldiers Reuters In pictures: Mosul offensive Women carry a boy over a wall as civilians flee their houses in the village of Tob Zawa, Iraq AP In pictures: Mosul offensive An Iraqi soldier and a civilian ride a motorbike as smoke rises behind them, on the road between Qayyarah and Mosul Getty In pictures: Mosul offensive A member of Iraqi forces, wearing a skull mask, waits at a checkpoint for people fleeing the main hub city of Mosul Getty In pictures: Mosul offensive An Iraqi soldier sits at a checkpoint in an area near Qayyarah Getty In pictures: Mosul offensive Iraqi men prepare food portions for Iraqi forces deployed in areas south of Mosul Getty In pictures: Mosul offensive Iraqi forces celebrate upon the arrival of vehicles bringing food to them Getty In pictures: Mosul offensive Iraqi childen smoke cigarettes upon the arrival of Iraqi forces in the village of Umm Mahahir, south of Mosul Getty In pictures: Mosul offensive A member of Iraqi forces distributes drinks to children in the village of Umm Mahahir, south of Mosul Getty

Iraqi forces repeatedly advanced too quickly into Mosul's eastern neighborhoods by day, only to face punishing counterattacks by night. However as Iraqi forces closed in on the Tigris River, they began to see swifter progress.

Iraqi and coalition officials claim this was due to new tactics and better coordination between the disparate forces fighting in Mosul, but Iraqi troops on the ground say Isis defenses simply began to thin, allowing them to secure swifter gains.

As in the fight for Mosul's east, Iraq's special forces are expected to take the lead in the battle for the west. The US-led coalition will continue to closely back Iraqi forces with both airstrikes and raids into Isis-held territory aimed at taking out key leaders and sowing unease in Isis-held neighborhoods.

Smoke rises after a rocket landed in the middle of the Iraqi rapid response forces' position during a battle against Isis militants in the south of Mosul, Iraq, 19 February, 2017 (Reuters)

There are now some 6,000 US troops in Iraq and last month was the first time the Pentagon confirmed that US forces are working inside Mosul. This is a sharp contrast to how the military engagement was originally cast more than two years ago.

When the US-led coalition began the fight against Isis in Iraq, Pentagon officials insisted there were no "American boots on the ground." Since then, US forces have steadily built up in Iraq and moved closer to front-line fighting.

Iraq's militarised federal police, US-trained rapid response units and the Iraqi army will also participate.

Air strikes destroy Islamic State drone base in Mosul

Iraq's government-sanctioned Popular Mobilization Units — mostly Shia militias — have held the western edge of Mosul's outskirts and have pledged to participate if their help is needed. In the past, Iraq's prime minister has sought to limit the involvement of PMU fighters in majority Sunni urban areas due to reports of abuses of civilians from human rights organisations.

Iraqi and coalition officials say the biggest difference in the fight for Mosul's west will be the terrain: the western half of the city is home to some of Mosul's oldest neighborhoods with narrow streets that Iraqi forces won't be able to drive armoured cars down. Even Iraq's special forces — some of the most competent fighters in Iraq's military — have so far largely fought the Mosul battle from inside their vehicles, rather than moving house-to-house on foot, in an effort to limit military casualties.

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Iraqi forces are expected to push into western Mosul from the south near the city's international airport. Originally, Iraqi forces planned to push in from the city's south as the fight for the east was unfolding, but as they became bogged down in eastern Mosul, the southern front was put on hold.

Iraq's militarised federal police retook the village of Hamam al-Alil in November and since then the front has barely moved as some units were dispatched to eastern Mosul to help with the fight there. On Sunday morning, shortly after the new push was announced, police units moved into the village of Athba, southwest of the city's Isis-held airport, while the army's 9th armoured division moved into the southwestern village of Bakhira.

Eastern Mosul was declared fully liberated in January, but has already experienced several militant attacks as Iraqi special forces pull out of the east and Iraqi army units, with less training and experience, take over the area.

Iraqi officials have warned that there is a high probability that small groups of Isis fighters have remained in Mosul's east and will continue to carry out attacks in neighborhoods declared liberated despite the front-line fight moving west.

The United Nations estimates that some 750,000 civilians are still living in Mosul's west and has described conditions there as "siege-like." Civilians who have escaped the western half of the city say food is running out as the wealthiest have stockpiled all the goods that were available in stores.

Aid organisations are so concerned by the humanitarian situation there, they are considering aid drops into Isis-held neighborhoods, according to a western diplomat speaking on condition of anonymity due to lack of authorisation to brief the press.