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Thousands of Iraqi and Kurdish troops, supported by the United States, France, and Britain, are now in the early stages of a massive operation to retake the Iraq's second largest city of Mosul from ISIS militants.

Advancing on multiple fronts, the Iraqi assault has so far recaptured numerous outlying villages, but central Mosul still lies ahead.

Members of the United Nations and others fear a major crisis may result as hundreds of thousands of civilians are driven from their homes by the war, while ISIS fighters have threatened to fight to the death. In villages already captured, booby traps are being discovered on roadsides, in the remaining buildings, and in networks of tunnels dug beneath them.

A man stands in front of an oil fire burning in the Qayyarah area, on October 19, 2016, during an operation by Iraqi forces against ISIS jihadists to retake the main hub city.

Iraqi Kurdish Peshmerga fighters fire a rocket launcher from a position in Sheikh Ali village near the town of Bashiqa, on October 20, 2016.

Smoke rises from burning oil wells on October 18, 2016 near the town of Qayyarah, south of Mosul.

Smoke rises from ISIS positions after an airstrike by coalition forces in Mosul, Iraq, on October 18, 2016. 

Kurdish Peshmerga soldiers rest close to their observation post in Nawaran, 20 kilometers northeast of Mosul, on October 20, 2016. 

Earth is piled up on a road to stop ISIS suicide bombers attacking Kurdish positions during the battle to retake Mosul, on October 19, 2016.

An Iraqi soldier protects himself from the dust as he sits on a vehicle heading to the front line in Kani al-Haramion October 19, 2016. 

Smoke rises as people flee their homes during clashes between Iraqi security forces and members of ISIS fleeing Mosul, Iraq, on October 18, 2016. 

A Kurdish Peshmerga fighter leans out of his military vehicle which has taken several direct hits from ISIS snipers including on the windscreen on October 18, 2016 in the small town of Bartella near Mosul. 

Members of Peshmerga forces inside a military vehicle north of Mosul, during an operation to attack ISIS on October 20, 2016. 

Iraqi forces sit in the back of a vehicle as troops advance through the desert, northeast of the main staging base of Qayyarah, on October 20, 2016. 

A member of Iraq's elite counterterrorism forces pauses as they advance towards the city of Mosul on October 20, 2016.

A Kurdish Peshmerga soldier walks through the rubble of a village previously held by ISIS and hit by a coalition airstrike on October 18, 2016.

An Iraqi Kurdish Peshmerga fighter stands inside of a damaged building on October 18, 2016 on the frontline in Shaqouli village.

A Kurdish Peshmerga soldier gestures towards a tunnel dug by ISIS fighters in a village recently recaptured by the Kurds on October 18, 2016.

A Kurdish Peshmerga soldier stands in the tunnel dug by ISIS.

Looking down into a tunnel dug by ISIS in a house recently recaptured by the Kurds, on October 18, 2016.

A Peshmerga fighter walks through the kitchen of an underground tunnel made by ISIS fighters on October 18, 2016. 

An Iraqi army soldier stands near the dead body of an ISIS fighter ( behind him) on the outskirts of Qayyarah, Iraq, on October 19, 2016. 

A picture taken on October 18, 2016 shows a helicopter prowling the perimeter of the Qayyarah military base, south of Mosul. 

An Iraqi forces M109 self-propelled howitzer fires towards the village of Tall al-Tibah, 30 kilometers south of Mosul, on October 19, 2016.

An Iraqi policeman tries on a gasmask at Qayyarah military base on October 16, 2016, as they prepare for an offensive to retake Mosul. 

Smoke rises as people flee their homes during clashes between Iraqi security forces and members of ISIS fleeing Mosul on October 18, 2016.

An Iraqi man walks on a street covered with smoke after oil was set ablaze in the Qayyarah area on October 19, 2016.

A man takes a selfie in front of an oil fire in the Qayyarah area, 60 kilometers (35 miles) south of Mosul, on October 19, 2016.

Displaced Iraqis from the Bajwaniyah village, who fled fighting in the Mosul area carry a white flag as they approach security forces on October 18.

A young Iraqi refugee, walks at a camp in the northeastern town of al-Hol in Syria's Hasakeh province on October 19, 2016.