There are even fears that they may unleash chemical weapons from the vast arsenal that was seized when they took over the city.
Iraqi troops, backed by the US-led coalition, are expected to allow a corridor that will let IS fighters flee into the desert towards Syria, where it will be easier to engage them.
However, allied commanders have prepared for weeks, if not months, of street-to-street fighting, which could lead to high casualties.
The Mosul military operation is one of the biggest in Iraq since the 2003 US-led invasion that toppled Saddam Hussein. “This is a decisive moment in the campaign to deliver ISIL a lasting defeat,” US defence secretary Ash Carter said.
“We are confident our Iraqi partners will prevail against our common enemy and free Mosul and Iraq from ISIL’s hatred and brutality.”
Mr Abadi sought to allay fears that the offensive would provoke sectarian bloodletting, saying only the Iraqi army and police would be allowed to enter the mainly Sunni city, not Shia militia.
Russian President Vladimir Putin said he hoped the US and its allies would do their best to avoid civilian casualties.
Foreign Secretary Boris Johnson urged Mr Putin to “pull the plug” on Syrian President Bashar Assad and end the bombardment of Aleppo.
In Luxem-bourg for talks with EU foreign ministers, Mr Johnson said they would be considering further economic sanctions and diplomatic pressure on Moscow.
However, talks in London on Sunday with US secretary of state John Kerry and other allies have made clear there is no appetite for Western military intervention in Syria, and Mr Johnson said: “The future salvation of Aleppo lies really with Assad regime and above all with the Russians.”
