Mosul, the last major stronghold of Islamic State fighters in Iraq, is five times the size of any other city the militant group has held.
Recapturing it would deal a decisive blow to its self-declared caliphate but the advance is expected to be difficult.
Tens of thousands of pro-government troops will face a tenacious enemy that has had years to prepare and a history of using civilians as human shields. A population of some 1.5 million will be in harm's way.
Iraqi government forces are mostly fighting south of Mosul with Kurdish Peshmerga fighters deployed on the eastern and northern frontlines, 20 to 25 km from the city.
An Iraqi army soldier told Reuters: “Spirits are high, the army is advancing. Daesh were killed, their bodies are left behind. Their bodies line the street. They are broken, defeated. The families (in the villages) are happy and relaxed. God willing a few days and Mosul will be in our hands.”
Iraqi officials and residents of Mosul say Islamic State is preventing people from leaving the city but civilians are fleeing from outlying districts and villages.
The United Nations says that in a worst case scenario 1 million people could be displaced by the Mosul campaign, requiring emergency housing and food. It has warned that some 100,000 people may arrive in Syria from the Mosul area.
REUTERS
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