Irbil, Iraq (CNN) Information filters out of Mosul in brief calls, smuggled messages and first-hand accounts from escapees. It's fragmentary and anecdotal, but a clear picture is emerging: ISIS leaders are preparing both for fierce battles and their own escape.

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.

ISIS bolsters defenses

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.

Photos: Mosul: Iraqi-led forces push into key city Members of the Iraqi federal police wave the country's flag as they celebrate in the Old City of Mosul on July 9, 2017. Iraq declared victory against ISIS forces in Mosul after a grueling monthslong campaign. The battle to reclaim Mosul, the last major ISIS stronghold in Iraq, has been underway since fall 2016. Hide Caption 1 of 73 Photos: Mosul: Iraqi-led forces push into key city This injured girl was found by Iraqi forces as they advanced against ISIS militants in the Old City of Mosul on Monday, July 3. She was carried away for medical assistance. Hide Caption 2 of 73 Photos: Mosul: Iraqi-led forces push into key city A suspected ISIS fighter is held in a basement while Iraqi forces continue to push for control of the Old City of Mosul on Monday, July 3. Hide Caption 3 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi Special Forces soldier exchanges fire with ISIS militants in the Old City on Friday, June 30. Hide Caption 4 of 73 Photos: Mosul: Iraqi-led forces push into key city A bomb explodes near the al-Nuri mosque complex on Thursday, June 29. Iraq's military has seized the remains of the Great Mosque of al-Nuri. Iraq and the United States have accused ISIS of blowing up the historic mosque. Hide Caption 5 of 73 Photos: Mosul: Iraqi-led forces push into key city Members of the Iraqi Federal Police hold a position as US-led coalition forces advance through the Old City on Wednesday, June 28. Hide Caption 6 of 73 Photos: Mosul: Iraqi-led forces push into key city The remnants of Mosul's ancient leaning minaret are seen in the Old City on Sunday, June 25. ISIS' claim that US warplanes were responsible for the destruction of the minaret is "1,000% false," US officials told CNN. Hide Caption 7 of 73 Photos: Mosul: Iraqi-led forces push into key city Two boys comfort each other after their home collapsed during fighting between Iraqi forces and militants in Mosul on Saturday, June 24. The boys, who are cousins, said some of their relatives were still under the rubble. Hide Caption 8 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi soldier helps transport a girl as residents flee their homes west of Mosul on Friday, May 26. Hide Caption 9 of 73 Photos: Mosul: Iraqi-led forces push into key city Displaced Iraqis make their way through rubble after evacuating their homes in a neighborhood of west Mosul on Wednesday, May 17. Hide Caption 10 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi man tries to extinguish a burning car during fighting in Mosul's western Rifai neighborhood on Tuesday, May 16. Hide Caption 11 of 73 Photos: Mosul: Iraqi-led forces push into key city A member of the Iraqi counterterrorism service secures a building as troops push toward Mosul's Al-Oraibi western district on Sunday, May 14. Hide Caption 12 of 73 Photos: Mosul: Iraqi-led forces push into key city A smoke cloud rises on the front line as the Iraqi Emergency Response Division advances in west Mosul on Saturday, May 6. Hide Caption 13 of 73 Photos: Mosul: Iraqi-led forces push into key city A wounded man is transported in western Mosul on Friday, April 21. Hide Caption 14 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi federal policeman smokes during a break from battle on Wednesday, April 12. Hide Caption 15 of 73 Photos: Mosul: Iraqi-led forces push into key city A member of Iraq's security forces stands guard in eastern Mosul as smoke rises from the ISIS-controlled western section of the city on Friday, April 7. Hide Caption 16 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqis visit a bath house on the southern outskirts of Mosul on Wednesday, April 5. Hide Caption 17 of 73 Photos: Mosul: Iraqi-led forces push into key city Flames billow from an explosion in Mosul during a clash between Iraqi forces and ISIS fighters on Sunday, March 5. Hide Caption 18 of 73 Photos: Mosul: Iraqi-led forces push into key city Families are forced to evacuate as Iraqi forces advance in western Mosul on Thursday, March 2. The number of internally displaced people has surged as the offensive effort has intensified. Hide Caption 19 of 73 Photos: Mosul: Iraqi-led forces push into key city Mosul residents cross a damaged bridge in the al-Sukkar neighborhood on Saturday, January 21. Hide Caption 20 of 73 Photos: Mosul: Iraqi-led forces push into key city French President Francois Hollande and French Defense Minister Jean-Yves Le Drian, right, view territory held by ISIS during a visit to a military outpost near Mosul on Monday, January 2. Hide Caption 21 of 73 Photos: Mosul: Iraqi-led forces push into key city A young girl takes part in a Christmas Day Mass at a church in the predominantly Christian town of Qaraqosh. The area's churches were heavily damaged by ISIS militants before the town was freed by Iraqi forces during the Mosul offensive. Hide Caption 22 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi Shiite fighters ride through a desert area near the village of Al-Boutha al-Sharqiyah, west of Mosul, on Friday, December 2. Hide Caption 23 of 73 Photos: Mosul: Iraqi-led forces push into key city Internally displaced Iraqis who fled the fighting in Mosul watch as a civilian drone films them at the al-Khazir camp on Thursday, December 1. Hide Caption 24 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi soldier searches a home for ISIS militants after Iraqi forces retook the village of Al-Qasr on Wednesday, November 30. Hide Caption 25 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi soldiers transport a comrade who was injured during a battle near the village of Haj Ali on Tuesday, November 29. Hide Caption 26 of 73 Photos: Mosul: Iraqi-led forces push into key city A man mourns during the funeral of four Iraqi paramilitary fighters who were killed in battles in the town of Tal Afar. Hide Caption 27 of 73 Photos: Mosul: Iraqi-led forces push into key city Displaced civilians return to the village of Tall Abtah on Friday, November 25, after Iraqi forces retook the village from ISIS. Hide Caption 28 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi civilians sit on the ground in Mosul on November 24. An Iraqi officer addressed the group, demanding to know the whereabouts of alleged ISIS militants who opened fire on troops a few days earlier. Hide Caption 29 of 73 Photos: Mosul: Iraqi-led forces push into key city An injured baby receives treatment at a field hospital in Mosul on November 15. Hide Caption 30 of 73 Photos: Mosul: Iraqi-led forces push into key city A woman cries Sunday, November 13, after seeing the St. Addai church that was damaged by ISIS fighters during their occupation of the Keramlis village. Hide Caption 31 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi special forces soldier prays next to a Humvee before troops pushed toward Mosul's Karkukli neighborhood on November 13. Hide Caption 32 of 73 Photos: Mosul: Iraqi-led forces push into key city A Kurdish Peshmerga fighter holds part of a defused bomb planted by ISIS militants in Bashiqa, Iraq, on Friday, November 11. Hide Caption 33 of 73 Photos: Mosul: Iraqi-led forces push into key city A member of Iraq's special forces guards two suspected ISIS fighters found hiding in a house in Mosul on November 11. Hide Caption 34 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi woman displaced by war holds her cat near a checkpoint in the Iraqi village of Shaqouli, east of Mosul, on November 10. Hide Caption 35 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi troops watch a broadcast of Donald Trump's acceptance speech in a house in Arbid, on the outskirts of Mosul, on Wednesday, November 9. Iraqi Prime Minister Haider al-Abadi congratulated Trump on his win and said he hoped for continued support in the war on ISIS. Hide Caption 36 of 73 Photos: Mosul: Iraqi-led forces push into key city US Marines install equipment at a coalition base in Qayyara on November 9. Hide Caption 37 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi forces member investigates a mass grave that was discovered after coalition forces recaptured the area of Hamam al-Alil on Monday, November 7. Hide Caption 38 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi children witness a man being interrogated by a member of the Iraqi army at a base next to the Al-Intissar neighborhood of Mosul on November 7. Hide Caption 39 of 73 Photos: Mosul: Iraqi-led forces push into key city A civilian man who fled the fighting trims his beard after reaching an Iraqi army position in Mosul on November 7. Hide Caption 40 of 73 Photos: Mosul: Iraqi-led forces push into key city Children play in debris created by an airstrike in Qayyara on Sunday, November 6. Hide Caption 41 of 73 Photos: Mosul: Iraqi-led forces push into key city Female members of the Freedom Party of Kurdistan sing as they hold a position near Bashiqa on November 6. Hide Caption 42 of 73 Photos: Mosul: Iraqi-led forces push into key city A baby is passed through a fence back to his mother at a refugee camp in the Khazir region on Saturday, November 5. Hide Caption 43 of 73 Photos: Mosul: Iraqi-led forces push into key city People line up to receive food at a refugee camp in the Khazir region on November 5. Thousands are taking refuge in camps set up for internally displaced people. Hide Caption 44 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi soldiers pass near a bridge destroyed in an airstrike in Qayyara on November 5. Hide Caption 45 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi soldiers patrol an alley on the outskirts of Mosul on Friday, November 4. Hide Caption 46 of 73 Photos: Mosul: Iraqi-led forces push into key city A suspected member of ISIS is detained at a checkpoint near Bartella, Iraq, on November 4. Hide Caption 47 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi families pack into a truck to be moved to camps on Thursday, November 3. Hide Caption 48 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi special forces soldier searches for the location of an ISIS sniper in Gogjali on November 1. Hide Caption 49 of 73 Photos: Mosul: Iraqi-led forces push into key city A man fleeing the village of Bazwaya carries a white flag as he arrives at a checkpoint on November 1. Hide Caption 50 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi soldier receives treatment after being injured during clashes with ISIS fighters near Bazwaya on October 31. Hide Caption 51 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi soldier navigates through a shattered windshield as coalition forces advance on Bazwaya on October 31. Hide Caption 52 of 73 Photos: Mosul: Iraqi-led forces push into key city Archbishop Yohanna Petros Mouche, center, performs Mass in the liberated town of Qaraqosh on Sunday, October 30. Hide Caption 53 of 73 Photos: Mosul: Iraqi-led forces push into key city Children play in a camp for internally displaced people near Kirkuk, Iraq, on October 30. More than 600 families from Tel Afar, a town west of Mosul, have been living in the camp since ISIS took control of the area in 2014. Hide Caption 54 of 73 Photos: Mosul: Iraqi-led forces push into key city Displaced families are seen on the road near Qayyara on Saturday, October 29. Hide Caption 55 of 73 Photos: Mosul: Iraqi-led forces push into key city U.S. military personnel take cover in a bunker after a mortar alarm was sounded at a coalition air base in Qayyara on Friday, October 28. Hide Caption 56 of 73 Photos: Mosul: Iraqi-led forces push into key city Women and children grieve over the grave of a family member at a Qayyara cemetery damaged by ISIS on October 27. Hide Caption 57 of 73 Photos: Mosul: Iraqi-led forces push into key city Residents of Qayyara wait for distribution of food and water rations on October 26. Local water sources have been contaminated by the burning oil and sulfur. Hide Caption 58 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraq's counterterrorism forces advance toward ISIS positions in Tob Zawa on October 25. Hide Caption 59 of 73 Photos: Mosul: Iraqi-led forces push into key city Soldiers give first aid to an injured boy in Tob Zawa on October 25. Hide Caption 60 of 73 Photos: Mosul: Iraqi-led forces push into key city Kurdish Peshmerga forces take positions as they start to move toward the Imam Reza and Tizxirab villages of the Bashiqa district on Sunday, October 23. Hide Caption 61 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi forces distribute fruit in the village of al-Khuwayn, south of Mosul, after recapturing it from ISIS on October 23. Hide Caption 62 of 73 Photos: Mosul: Iraqi-led forces push into key city Kurdish security forces detain a suspected member of ISIS in the eastern suburbs of Kirkuk on Saturday, October 22. Hide Caption 63 of 73 Photos: Mosul: Iraqi-led forces push into key city An Iraqi forces member helps a man push a car as they arrive at a refugee camp in Qayyara on October 22. Hide Caption 64 of 73 Photos: Mosul: Iraqi-led forces push into key city Spent bullet cartridges litter the street around the Jihad Hotel, where ISIS militants battled Iraqi security forces in Kirkuk on Friday, October 21. Hide Caption 65 of 73 Photos: Mosul: Iraqi-led forces push into key city Gen. Abdel Ghani al-Asadi, who leads Iraq's counterterrorism forces, sits in Bartella on October 21 after the town was reclaimed. Hide Caption 66 of 73 Photos: Mosul: Iraqi-led forces push into key city Peshmerga fighters look over a village during an assault near Bashiqa on Thursday, October 20. Hide Caption 67 of 73 Photos: Mosul: Iraqi-led forces push into key city Iraqi forces head toward the front lines near Qayyara on Tuesday, October 18. Hide Caption 68 of 73 Photos: Mosul: Iraqi-led forces push into key city A Peshmerga fighter peers up from an underground tunnel in the liberated town of Badana on October 18. ISIS fighters have built tunnels below residential streets to escape from airstrikes. Hide Caption 69 of 73 Photos: Mosul: Iraqi-led forces push into key city Kurdish security forces take up a position near ISIS-controlled villages on Monday, October 17. Hide Caption 70 of 73 Photos: Mosul: Iraqi-led forces push into key city Smoke rises from a suicide car bomb attack carried out by ISIS in the village of Bedene on October 17. Hide Caption 71 of 73 Photos: Mosul: Iraqi-led forces push into key city Members of the Iraqi coalition gather around a fire at Zardak mountain ahead of the offensive. Hide Caption 72 of 73 Photos: Mosul: Iraqi-led forces push into key city Peshmerga forces deploy in the dark near the village of Wardak early on October 17. Hide Caption 73 of 73

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.

Tunnel networks

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.

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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.

Resistance

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.

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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.)

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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.