BAGHDAD (AP) — 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 the Islamic State group 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 grueling urban combat with high civilian and military casualties. Iraqi and U.S.-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.

THE OPERATION SO FAR

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.

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 IS defenses simply began to thin, allowing them to secure swifter gains.

THE PLAYERS

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 U.S.-led coalition will continue to closely back Iraqi forces with both airstrikes and raids into IS-held territory aimed at taking out key leaders and sowing unease in IS-held neighborhoods.

There are now some 6,000 U.S. troops in Iraq and last month was the first time the Pentagon confirmed that U.S. 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 U.S.-led coalition began the fight against IS in Iraq, Pentagon officials insisted there were no "American boots on the ground." Since then, U.S. forces have steadily built up in Iraq and moved closer to front-line fighting.

Iraq's militarized federal police, U.S.-trained rapid response units and the Iraqi army will also participate.

Iraq's government-sanctioned Popular Mobilization Units — mostly Shiite 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 involvements of PMU fighters in majority Sunni urban areas due to reports of abuses of civilians from human rights organizations.

THE TERRAIN

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 armored 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.

SOUTHERN FRONT

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 militarized 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 IS-held airport, while the army's 9th armored division moved into the southwestern village of Bakhira.

MILITANT THREAT

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 IS 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.

HUMANITARIAN CONCERNS

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