The U.S.-backed Iraqi mission to retake the besieged western side of Mosul began Sunday, a day before President Donald Trump’s U.S Pentagon chief met with Iraqi officials to discuss the ongoing battle to defeat the jihadists.

“Iraqi forces advanced Monday into the southern outskirts of Mosul on the second day of a push to drive Islamic State militants from the city’s western half, as the visiting U.S. defense secretary met with officials to discuss the fight against the extremists,” reports the Associated Press (AP).

U.S. Secretary of Defense James Mattis arrived in Baghdad on Monday on an unexpected visit. He has been touring the region in recent days.

Since 2014, the Islamic State (ISIS/ISIL) has held northern Iraq’s Mosul, the country’s second-largest city, and the jihadists last major urban bastion.

Tens of thousand of U.S.-backed Iraqi military troops, Kurdish Peshmerga forces, Iran-allied Shiite militias, and Sunni tribesmen, among other groups, formed an alliance that has been fighting to recapture Mosul from ISIS since mid-October.

Backed by U.S.-led airstrikes, U.S. special operations forces on the ground and the thousands of American troops providing support, Iraqi police and army troops launched the offensive to recapture western Mosul, first targeting the Abu Saif village.

On Monday, they entered the village and seized strategic territory within it.

“Separately, militarized police in armored vehicles were moving toward the sprawling Ghazlani military base on the southwestern outskirts of the city,” notes AP.

The U.S.-backed Iraqi government forces and allies announced the liberation of eastern Mosul in January. Some news reports and witnesses suggest that life is beginning to return to some parts of the ancient Sunni-majority city, once home to the largest concentration of Iraqi minority groups, such as Yazidis and Christians.

When compared to retaking the eastern side of the Tigris city that divides Mosul, top military officials and analysts predict the battle to be tougher and the deadliest to date, according to AP.

The news agency adds:

The battle for western Mosul, the extremist group’s last major urban bastion in Iraq, is expected to be the most daunting yet.

The streets are older and narrower in that sector of the city, which stretches west from the Tigris River, forcing Iraqi soldiers to leave the relative safety of their armored vehicles. The presence of up to 750,000 civilians also poses a challenge.

An American intelligence official told reporters on condition of anonymity during Matti’s recent trip to Iraq that “there’s about 2,000 [ISIS jihadists] remaining” in Mosul, down from up to 7,000 at the start of the offensive in mid-October.

Moreover, Iraqi Lt. Gen. Abdul Ghani al-Assad told Reuters last month that the U.S.-backed Iraqi government forces and their allies have killed the majority of ISIS commanders in Mosul.