Earlier this year, Al Qaeda's leader repeated criticism of the Islamic State's violent attacks, such as beheadings of civilians.
Several predicted deadlines for victory have come and gone, and the fighting is now largely centered in the "Old City" area of western Mosul, which Iraqi troops arrived in about a month ago.
Iraqi Federal Police forces "are engaged in hard, house-to-house clashes with ISIS fighters inside the Old City", a media officer from these units told Reuters.
"The battle in western Mosul is very different than in the east - it's much tougher".
The operation to retake western Mosul is complicated because of its population density and narrow, congested streets.
Some 20,000 people have escaped from Mosul in the past four days, fewer than before due to the lack of transport, the UNHCR said in a report.
The Iraqi government force's advance toward Mosul came after the Iraqi Prime Minister Haider al-Abadi announced on February 19 the start of an offensive to drive the extremist militants out of the western side of Mosul, locally known as the right bank of Tigris River which bisects the city.
The United Nations says almost half a million civilians had fled fighting since the offensive to retake Mosul from Daesh terrorists started on October 17, 2016.
According to estimates, as many as 500,000 people still remain in ISIL-controlled areas in western Mosul, including about 400,000 in the densely populated old city.
Heavy exchanges of gunfire and mortar rounds could be heard from the neighborhoods facing the old city across the Tigris River that bisects Mosul into a western and eastern sides.
"They (Islamic State militants) carry out attacks on our defensive lines, but each time we repel them and they run away, leaving bodies of their dead fighters behind", Lieutenant Colonel Hussein Lazim Zghayer said of the force's 9th division. "There are discussions and a constant dialogue between the messengers of Abu Bakr al-Baghdadi and Ayman al-Zawahiri", added the Iraqi leader.
ISIS and Al-Qaeda are planning to become one, declared the Iraqi vice-president Ayad Allawi..
It is unclear how the two groups would work together, Allawi said.