The northern Iraqi city of Mosul has been totally surrounded by US-backed troops, the Pentagon confirmed on Wednesday (7 December).

An offensive to capture the populous city from Islamic State (IS) has nearly lasted a period of two months, however a spokesman said Mosul's fall was now "inevitable".

The charge is being led by Iraq's Popular Mobilisation Forces (PMF) and Counter-Terrorism Service (CTS), which are made up of roughly 40 militias that are mainly made up of Shiite Muslim groups but also include Sunni Muslim, Christian and Yazidi fighters. It is estimated roughly 100,000 fighters are involved.

Pentagon spokesman Navy Captain Jeff Davis said the heaviest fighting had taken place in the east, east of the Tigris River, but the whole city was now under siege.

"Mosul is effectively isolated now," Davis said.

"They [IS (Deash)] still have freedom of movement between Mosul and Tal Afar, but they don't have freedom of movement beyond Tal Afar and into Syria."

Despite the early victories, the fighting is expected to be much more difficult here on for Iraqi and US forces, as they enter more built-up areas of the city, which are easier to defend.

"This is hard fighting, but the fact remains that ISIL is surrounded by a superior force," Davis said, using an alternative name for IS.

"They're facing resistance from within the city, they're being bombarded daily by coalition air and artillery strikes, they have no ability to resupply or reinforce, and it is a military fact that they are in a position where defeat is inevitable."

The UN believes the fight for Mosul to be a "humanitarian catastrophe" as 700,000 people are expected to be displaced.

That has since been exacerbated as it was confirmed last week that the water supply for 650,000 people of the city's population of 1.5 million was completely cut off, after a pipeline was struck during fighting.

"The maintenance team cannot reach the pipeline because it lies in an area being fought over," Hussam al-Abar, a member of Mosul's Nineveh provincial council, told Reuters.

"Basic services such as water, electricity, health, food are non-existent [in Mosul].

"We need the help of international organisations for the hospitals. We have people wounded from suicide bombings and rocket attacks from Daesh who need treatment."

Davis added: "This is a large, crowded, heavily populated city, and the best way to go about achieving a sustainable victory is to be very slow and methodical."