MOSUL, IRAQ — (CNN) One week into the battle for Mosul, and the vast coalition seeking to oust ISIS from Iraq’s second city is making swifter than expected progress.

But for all its gains — 78 villages liberated, and nearly 800 ISIS fighters killed as of Monday morning — the coalition is encountering fierce resistance from ISIS, in what is anticipated to be the terror group’s last stand in Iraq.

Massively outnumbered by the advancing coalition — a 90,000-strong force of Iraqi government troops, Kurdish Peshmerga fighters and irregular militia soldiers — ISIS relies on asymmetric warfare tactics to inflict damage on its opponents, and terrorize long-suffering local civilians.

Here’s how the terror group is fighting to maintain its two-year grip on the city.

Lighting toxic fires

ISIS has also been setting fire to oil wells in the oil-rich region, in an attempt to blunt the effectiveness of the coalition’s air power by obscuring their view of targets from above.

A sulfur facility near Qayyara was set ablaze Thursday, by ISIS militants who placed explosives and oil in the sulfur deposits and across the factory.

The fire has burned for days, emitting toxic plumes that have driven hundreds to seek medical treatment, and complicating military operations in the area.

Bombs

Having known long in advance that the push to retake Mosul was coming, ISIS has had plenty of time to prepare for the onslaught.

Coalition forces approaching the city must navigate roads lined with improvised explosive devices (IEDs). In recaptured towns, they face the threat of booby traps left by ISIS’s skilled bombmakers.

According to the Iraqi Joint Operations Command, two bomb-making factories were discovered during the first week of the assault, and nearly 400 IEDs remotely detonated by coalition forces.

Brigadier General Bajat Mzuri of the Zeravani Special Forces, part of the Kurdish Peshmerga, told CNN that more of his troops had been killed by such explosives than on the battlefield itself.

“They put them on the road, in the houses,” he said. “We liberate a village and they are everywhere — people come back to their homes, open a door or even a refrigerator and it blows up.”

Clearing a village of ISIS IEDS is slow, dangerous work that can take months, he said. A third of his team’s casualties to IEDs are bomb specialists brought in to eradicate the threat.

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Suicide attacks

Suicide attacks — whether via car or truck bombs, or gunmen wearing suicide vests — have become one of ISIS’ signature modes of attack. CNN teams in the field have witnessed multiple attempted suicide attacks.

Iraqi military officials said Monday that 127 vehicle-borne improvised explosive devices (VBIEDs) had been destroyed in the first week of fighting.

Peshmerga officials gave CNN a up-close look at a suicide device that had been recovered from ISIS fighters and defused before it could be detonated: a silver box packed with C4 explosives and hundreds of ball bearings, carried in a backpack.

The use of suicide vests allows even small pockets of fighters to tie up advancing Iraqi forces, detonating their suicide belts to potentially inflict mass casualties once they are shot.

Tunnels

A key part of ISIS’s apparent defense strategy for Mosul has been the construction of an elaborate network of tunnels in the city and surrounding towns.

This concealed network allows fighters to escape, hide, move equipment and personnel without detection from coalition aircraft — and launch surprise attacks on passing coalition troops.

CNN correspondents have witnessed ISIS fighters suddenly emerge from the ground to fire on coalition forces as they press towards Mosul. Even a single fighter — especially one wearing a suicide vest — can present a threat.

The Peshmerga say that in some tunnels, they have found plates of food that are still warm — indicating how close coalition forces come to the terrorists before they flee.

Distraction assaults

As the coalition closes in on Mosul, ISIS has launched surprise attacks in other parts of the country, in an effort to distract coalition forces and tie up their attention and resources elsewhere.

A major attack Friday on Kirkuk, about 175 kilometers to Mosul’s southeast, was followed by another assault south of the city Sunday.

The same day, ISIS sleeper cells launched a surprise assault on Rutba, a town in Anbar province hundreds of kilometers to the southwest of Mosul, which had been recaptured by Iraqi forces from ISIS in May.

As well as drawing the Iraqi military’s focus and resources away from Mosul, the surprise attacks demonstrate the enduring ability of ISIS sleeper cells to strike deep behind the frontlines — and underline the enduring vulnerability of supposedly recaptured territory.

Retribution killings

Nowhere was this point more apparent than in a “liberated” village near Nimrud, southeast of Mosul, where about 40 people who had welcomed Iraqi forces were reportedly executed by ISIS on Saturday night.

People in the village said that ISIS fighters had hid when Iraqi forces passed through, a former Mosul City Council official told CNN.

The coalition forces did not leave troops behind to secure the town, allowing the extremists to return and take bloody revenge on locals who had celebrated their defeat once the troops moved on.

ISIS has reportedly retaken a number of “liberated” villages in recent days, capitalizing on the coalition’s failure to secure all of its gains on the battlefield.

Human Shields

There are also fears that the terror group is targeting civilian populations by using them as human shields against the coalition advance, as it has done in previous battles in Iraq.

An Iraqi intelligence source told CNN that 284 men and boys were executed in Mosul on Thursday and Friday, after they had been taken for use as human shields. CNN could not independently confirm the claim.

The United Nations earlier said it was concerned by reports that ISIS had taken 550 families from villages around Mosul as human shields, as part of an apparent policy to prevent civilians escaping.
Zeid Ra’ad Al Hussein, UN High Commissioner for Human Rights, said there was a grave danger that ISIS would “not only use such vulnerable people as human shields, but may opt to kill them rather than see them liberated.”

Drones

Long targeted by the US’s sophisticated drone technology, which has successfully taken out many of the group’s leaders, ISIS now appears to be deploying crude unmanned aerial vehicles of its own on the battlefield.

While they offer no rebuttal to the devastating aerial power of the coalition, the drones allow ISIS to get above their opponents — to drop crude explosives, or spot targets for artillery fire.

A CNN team embedded with the Peshmerga fighters pushing towards Mosul saw a drone flying towards the Kurdish position, before it was gunned down. The Peshmerga say the drones are being used to observe their positions, or drop grenade-like bombs.

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