There are also stirrings of civilian resistance as conditions for the population become ever more intolerable.
It can take several hours of phone calls to get a few scraps of information on poor connections, sometimes against a background of explosions. People are terrified of being caught with cell phones. ISIS has announced that anyone found with a phone will be summarily executed.
But still, a courageous few take the risk to tell the outside world what is happening inside Mosul.
Witnesses in Mosul told CNN that several hundred ISIS fighters have arrived in the city in the past few days from Raqqa
in neighboring Syria. They say most of the new arrivals are foreign fighters who wear distinct uniforms. They are seen wearing suicide belts for show -- and carry light weapons.
These fighters appear to have moved to Mosul in small groups to avoid airstrikes, taking a long route to the western Iraqi town of Ba'aj and then across the desert on small roads and tracks.
Western intelligence estimates there are between 3,000 and 5,000 ISIS militants in the city, with hundreds more in areas around it.
Witnesses report intense defensive preparations -- with the movement of vehicles to the eastern and southern outskirts, where the main brunt of the offensive is expected. The assumption is that these vehicles will become mobile suicide bombs. Witnesses also told CNN that rocket systems had been moved to the east of the city.
Others say that ISIS has booby-trapped dozens of empty houses in the Hadbaa neighborhood of northeastern Mosul. The terror group had given local residents the option to stay or leave the neighborhood. Piles of tires had been set ablaze in the area, sending thick columns of black smoke skyward, to disguise the movements of ISIS fighters.
There are thousands of abandoned homes across the city that the group can use for deploying fighters and setting booby traps. ISIS has long used sophisticated ploys to detonate IEDs, using light switches or even fridge doors as triggers.
But the coalition appears to have intelligence on some of ISIS' moves inside Mosul. At dawn on Thursday, residents said, an airstrike destroyed three houses that were occupied by an ISIS squad trained to use anti-aircraft and tank weapons. The witnesses said the fighters had been training on the weapons for two weeks; about 20 were killed in the strike.
According to people who are in or have escaped from Mosul, there is an elaborate network of tunnels across the city. These people know because they've been forced to dig them.
One resident told CNN that men caught smoking would be sentenced to dig 10 meters of tunnel. Men wearing non-Islamic dress or with shaved beards would be given a choice: lashes, prison or labor underground. Those that choose digging are blindfolded and taken to a tunnel project.
The tunnels begin in the eastern suburbs and stretch for several kilometers -- sometimes connecting under a building. They are not deep underground, but some are big enough for a motorbike. In one area, according to a witness, ISIS even built a new road to connect two tunnel entrances.
A former security officer who fled Mosul in 2015 and current residents say ISIS has built two kinds of tunnels: some to recover the city's rich antiquities and sell them; others to assist the movements of the organization's military network.
Preparing to escape?
At the same time as they bolster defenses, militants appear to be preparing to escape or melt into the civilian population. Several ISIS militants who are well known in two neighborhoods in central Mosul have shaved their beards, according to residents. They are members of ISIS' police force -- al Hasba.
One resident, contacted by the French news agency AFP by phone, said, "I saw some Daesh (ISIS) members and they looked completely different from the last time I saw them."
There are also multiple reports that fighters have been moving from the east to the west of the city across the River Tigris. Witnesses say that ISIS has begun rigging bridges across the river with explosives, which they may detonate to prevent opposing forces from crossing the river.
The west has old neighborhoods with tightly packed houses and narrow alleys -- perfect defensive territory for ISIS. It's also closer to escape routes across the desert toward Syria. Iraqi and Western analysts expect a hard core of fighters to remain behind and fight to the death, among them foreign fighters, but many among the leadership will try to make it to Raqqa.
According to the offensive battle plan, the Shia Hashd al Shabi militia, also known as the Popular Mobilization Unit,will be tasked with blocking any escape of ISIS militants. Mindful that areas west of the city could become another battlefield, aid agencies have urged civilians not to flee in that direction.
For several months there have been sporadic assassinations of ISIS fighters, often at night, but not widespread resistance. This may be changing. In the early hours of Monday, residents heard gunfire for about an hour in the southern neighborhood of Wadi Hajer, near the airport.
ISIS opened fire indiscriminately on houses and rooftops as snipers in the budding resistance movement targeted the militants before escaping across the rooftops. Wadi Hajer is a warren of alleys and small houses well-suited to hit-and-run operations.
At least five ISIS militants were killed in the clashes and ISIS was swift to exact retribution. It sealed off the neighborhood and began arrests. So far, according to witnesses, at least 600 people have been detained, including former members of the Iraqi security services and civilians. Some 50 people were executed, witnesses say, several of them outside their own homes.
ISIS has launched overnight house raids against anyone suspected of links to the resistance, using the feared al Hasba police, and had stepped up patrols with pick-ups mounted with heavy machine guns.
Former residents of Mosul say they expect resistance to grow as the offensive nears the gates of Mosul but are concerned that ISIS has had too much time to prepare defensive positions. The main resistance group is known as Kataib al-Mosul, or the Mosul Battalions.
Residents have few sources of information about the progress of the offensive. Those who dare listen to a radio station broadcasting from Irbil
, an offense punishable by death.
Horror of ISIS occupation
Those who have been able to escape tell horrendous stories of ISIS' occupation. One woman told a Kurdish TV network that she'd taken her sister to a hospital in Mosul. She'd met a young Yazidi girl with a child who had begged for help to escape. The girl said the father of the child was an ISIS emir (leader.)
Families have been separated. One elderly woman inside Mosul told CNN: "I have illnesses. I am scared death will come and I miss my children." Her sons fled when ISIS arrived; they could have no idea that more than two years later it would still hold Iraq's second largest city.
Others speak of their fears for the future: the rampant distrust in the city as people wonder who acted as ISIS informants, revenge attacks and a spike in sectarian hatred. One man inside Mosul told CNN last week that he was scared of the Peshmerga asking him why he didn't leave ISIS-controlled areas. But as a Sunni, he was also afraid of retaliation by Shia.
Fear of being caught in the middle of a brutal war for the city preoccupies many residents now -- but they are also worried by what will follow.