ERBIL, Iraq — Iraqi forces opened a new front Thursday in their offensive to retake Mosul, advancing from the northwest in a bid to add momentum to the grueling seven-month-long battle for the city.

Troops from the army’s 9th Division and elite forces from the Interior Ministry spearheaded the attack, according to the Iraqi military.

Islamic State militants are still holding on in a few neighborhoods of western Mosul, after losing control of the east. But the Iraqi offensive, backed by airstrikes and military support from a US-led coalition, had slowed in recent weeks. Interior Ministry forces have ground to a halt on the southern edge of Mosul’s Old City, whose narrow streets and winding alleys are inaccessible to armored vehicles.

Given that challenging geography, Iraqi commanders have said they plan to lay siege to the Old City by attacking from the northwest. But the district remains home to hundreds of thousands of civilians, raising concerns about their welfare as the battle closes in, and food and drinking water become increasingly scarce.

‘‘The Old City will be isolated and the last target,’’ said Lieutenant General Sami al-Aridhi, a counterterrorism forces commander. He said the authorities are considering dropping humanitarian aid to families trapped in areas held by the Islamic State.

Nearly half a million people have been displaced since the battle for Mosul began in October, the United Nations says.

After a grinding fight, during which Iraqi forces suffered heavy casualties, commanders are eager to wrap up the battle before the Muslim holy month of Ramadan begins.

The Islamic State’s capability ‘‘continues to wane,’’ said Major General Joseph Martin, commander of ground forces for the US-led coalition. The security forces have made ‘‘undeniable progress,’’ he added.

But the presence of civilians makes it harder to push forward at a steady pace.