Iraqi forces, backed by heavy U.S.-led air and artillery strikes, began a new offensive into the Islamic State’s final bastions in the city.

The operation, announced Saturday by Iraq’s Joint Operations Command, comes after Iraqi troops have cleared most of the western parts of Mosul, leaving a small pocket of Islamic State fighters spread across a handful of neighborhoods along the western bank of the Tigris River.

The attack began at dawn and involved several Iraqi units assaulting across a broad front in an effort to overwhelm the Islamic State’s defenses.

After nearly eight months of fighting, Iraqi forces appear poised to retake what was once the Islamic State’s largest stronghold in Iraq and the birthplace of its caliphate. Fighting, especially in the city’s west, has been ferocious, leaving Iraqi forces with thousands of casualties. Civilians, too, have been caught in the crossfire, leaving hundreds of thousands displaced and thousands dead.

Even though the Islamic State now controls only roughly five square miles of the city, Iraqi and U.S. commanders have cautioned that the worst fighting is yet to come.

From his small patrol base north of the city, U.S. Army Capt. Devan Zimmerman said the U.S.-led coalition had already provided a significant number of air and artillery strikes Saturday morning.

Yet with such a small amount of the city still held by the militants and so many Iraqi forces attacking at once, Zimmerman said, it will be difficult to marshal and coordinate U.S.-led aircraft over the city.

“This isn’t going to be quick,” said Zimmerman, who helps advise a brigade from the Iraqi 9th Division.

Despite potential early gains, Iraqi forces have taken casualties. Late Saturday, the Iraqi Joint Operations Command issued a statement that two colonels from the army’s 16th Division were killed in the fighting.

Along with the militants still in parts of the city, tens of thousands of Iraqi civilians remain trapped in their homes, creating the potential for a humanitarian catastrophe if they are not evacuated safely.

On Friday, Iraqi forces dropped thousands of leaflets over western Mosul, telling those in their homes to flee. The move is an abrupt shift from past instructions that have mostly told civilians to stay in place until the fighting is over. Aid groups cautioned the Iraqi government, saying that its forces must ensure that escape routes are protected before civilians leave.

On Saturday, checkpoints outside the city saw a surge in civilians trying to get out of the city. The United Nations has estimated that roughly 200,000 civilians remain in Islamic State-occupied neighborhoods and that 10,000 have begun leaving each day.