Here's what analysts at IHS Country Risk, Zaineb Al-Assam and Columb Strack, have to say about how long the offensive could carry on for:

Most Iraqis anticipate that the operation will be a long and difficult one potentially spanning months.

The Islamic State will be asking themselves how much of their dwindling resources they are willing to commit to a losing battle, at a time when they face an existential threat to the core of their Caliphate in Syria.

Last week, reports emerged of a convoy of IS militants and their families from Mosul arriving in Raqqa.

Back in July 2016, Iraqi news reports claimed 1500 IS members including senior leadership had fled to Syria soon after the recapture of Qayyarah airbase.

However, Islamic State militants can utilise the network of tunnels it has built in preparation for the offensive in order to put up a fierce fight.

Mosul has been the epicentre of the Islamic State’s chemical weapons capability. They have frequently used chlorine and mustard agent against the Kurds and the Iraqi Security Forces around Mosul over the last year, and chemical weapons are likely to play a role in their defence of the city.

The bulk of the Islamic State’s forces in Mosul are locals, many of which have been conscripted against their will.

Order is maintained by a relatively small number of ideologically committed foreign fighters under threat.

The ability of these foreign fighters to maintain order among the ranks and to prevent defections will be a key determinant of how long the battle is likely to take.

As Iraqi forces close in on Mosul, this may embolden local Mosul resistance including the Mosul Brigades who have only been able to mount a very limited- albeit symbolic resistance- to Islamic State prior to the operation.

In the coming weeks/months as the fight enters urban areas, their participation may prove invaluable in providing intelligence on Islamic State whereabouts.