Mosul was where IS supremo Abu Bakr al-Baghdadi declared his "caliphate" two years ago but is now the group's last major stronghold in Iraq.
Iraqi Prime Minister Haider al-Abadi, who announced the launch of a broad offensive to retake the city on Monday, visited the front line today. In the biggest Iraqi military operation in years, forces have retaken dozens of villages, mostly south and east of Mosul, and are planning multiple assaults for Thursday.
"We are telling Daesh (IS) that their leaders are abandoning them. We've seen a movement out of Mosul," said Major General Gary Volesky, who heads the anti-IS coalition's land component.
He told reporters in a video briefing that the many foreigners among the 3,000 to 4,500 IS fighters would likely end up forming the core of the holdout jihadist force.
Volesky noted that the Iraqis would screen anyone leaving Mosul, and attempts by foreign fighters to blend in to an expected exodus of displaced people would be thwarted. "It's difficult for them to blend into the local population based on the number of different types of foreign fighters that there are," he said.
Hundreds of thousands of civilians were still trapped in the city with dwindling supplies, many sheltering in basements as air strikes intensified on IS targets. "We couldn't sleep last night because of the air strikes.
The explosions were huge," said Abu Saif, a 47-year-old resident contacted by AFP. "Many families are starting to run out of some basic food goods, there is no commercial activity in Mosul -- the city is cut off from the world," he said.
East of Mosul, forces were poised for an assault on Qaraqosh, which lies about 15 kilometres away and was once Iraq's largest Christian town. News of the move to recapture Qaraqosh sparked jubilation among Christians who had fled the town, with many dancing and singing in the Iraqi Kurdish city of Arbil on Tuesday night.
Units from Iraq's elite counter-terrorism service, which has done the heavy lifting in most recent operations against IS, were poised to flush jihadists out of the town, officers said. "We are surrounding Hamdaniya now," Lieutenant General Riyadh Tawfiq, commander of Iraq's ground forces, told AFP at the main staging base of Qayyarah, referring to the district that includes Qaraqosh.
"There are some pockets (of resistance), some clashes, they send car bombs -- but it will not help them," he said. Qaraqosh was the largest of many Christian towns and villages seized by the jihadists who swept across the Nineveh plain east of Mosul in August 2014.