The elite special forces have mastered the killing of suicide car bombers and have acted as ground spotters for coalition air strikes.
They also backed up at least 50,000 Iraqi soldiers, police and paramilitaries advancing on the city from the south and west. Four US troops have died in combat in Iraq since they were deployed there in 2014.
Haider al-Abadi, the Iraqi Prime Minister said: “Our forces are beginning the liberation of the citizens from the terror of Isis.”
There are an estimated 4,000 jihadi fighters defending the close-packed houses and narrow alleyways in the half of the city west of the Tigris River, which is inhabited by some 650,000 civilians.
Around 80 per cent of residents are said to be without running water, and the UN said bakeries and half of food shops had closed for lack of flour and fuel.
Lieutenant-General Stephen Townsend, the American head of the western anti-ISIS coalition, said: “Mosul would be a tough fight for any army in the world.”
“The Iraqi forces have risen to the challenge. They have taken the fight to the enemy and sacrificed their blood for the people of Iraq and the rest of the world.”
They will also be strongly supported by US artillery and airstrikes seeking to eliminate Isis strongpoints.
An Iraqi officer who gave his name as Captain Ali said: “The villages were empty.”
“Isis forced most of the civilians back to west Mosul as human shields.”
Deash forces are believed to have been heavily weakened by casualties, with as many as 1,500 killed in east Mosul. The city is now effectively surrounded, making resupply no longer possible.
Captain Ali believes the real fighting will start within the city. He said: “West Mosul is their last stand in Iraq, it will be harder than the east, worse than Fallujah.”
The terrorists have recently depended on mobile squads of snipers, booby traps and over 600 suicide bombers driving vehicles packed with explosives to try and slow the advance of the coalition army.
General Raad Shaker Jawdat, head of the police units involved, said his men had destroyed 13 car bombs and three tunnels, defused 30 mines and seized five suicide belts along with several weapons caches.