After securing the eastern part of the megapolis earlier this year, fighting in heavily populated west Mosul was expected to turn into a tough challenge for Iraqi forces due to the city's narrow alleyways and streets which does not allow for armored vehicles and tanks to go through.
In Iraq, flooding in Tigris River has cut off aid supplies and escape routes for people fleeing Islamic State militants in Mosul city.
US-backed Iraqi forces are now battling IS militants in the more densely-populated western half of Mosul, with more than half of that area having been retaken.
The attack occurred in an area of western Mosul that has been recently liberated from the militants by US-backed Iraqi forces, AP reported on Saturday, citing an Iraqi military officer who spoke on condition of anonymity.
"ISIS suicide motorcycles now are their favorite weapon inside the Old City", he said.
The war between ISIS militants and Iraqi forces is taking a heavy toll on several hundred thousand civilians trapped inside the city, with severely malnourished babies reaching hospitals in government-held areas.
"You can not get vehicles in there, so it's gotta be a dismounted operation", Brigadier General Rick Uribe, a senior coalition commander, said of the Old City.
As many as 500,000 civilians could remain in parts of western Mosul under Daesh jihadist group control, OCHA estimates.
Meanwhile, the federal police and interior ministry special forces, known as Rapid Response, continued their battles on the southern and western front lines at the edge of the old city center, the source said.
In a related development Thursday night, the Iraqi army announced the capture of Mosul's main western entrance.
He said that around 30 people had been killed in the attack.