US pledges support for "historic operation" amid fears for fate of civilians trapped in ISIL’s de-facto capital in Iraq.
Iraqi government forces have launched a campaign to retake Mosul, the de-facto capital of the Islamic State of Iraq and the Levant (ISIL) group in Iraq.
Up to 1.5 million civilians remain in the city, according to the United Nations, amid fears that the vastly outnumbered ISIL fighters could use them as human shields as they seek to repel the assault on its last major stronghold in the country.
Mosul is Iraq’s second largest city and the last urban centre still under ISIL control in Iraq after a series of government offensives to reverse the group’s seizure of territory in 2014.
Al Jazeera’s Zeina Khodr in Khazir, one of the frontline positions around the ISIL-controlled city of Mosul
This is a very complicated operation, simply because of the mix of forces that are taking part. There is the central government in Baghdad, the Iraqi forces, Iraqi counterterrorism units and there is also the Kurdish Peshmerga who are allied in this fight but who do have a lot of differences.
There is also the question of Iranian-backed Shia militias - a very controversial issue because the people of Mosul are mainly Sunni. They fear that if the Shia militias actually take part and enter the city there will be reprisals. But what we understand from the government is that they are going to be staying at the perimeter of Mosul and they will not be advancing towards the city centre.
"The hour has come and the moment of great victory is near," Haider al-Abadi, Iraq’s prime minister, said early on Monday in a speech broadcast on state TV, surrounded by the armed forces’ top commanders.
The bid to retake Mosul comes after the military, backed by armed tribes, militias and US-led coalition air strikes, regained much of the territory the fighters seized in 2014 and 2015.
"We are proud to stand with you in this historic operation," Brett McGurk, US envoy to the coalition against ISIL, said on Twitter at the start of the Mosul offensive.
Al Jazeera’s Zeina Khodr, reporting from Khazir, just east of Mosul, said preparations for the offensive had long been under way, with forces amassing around the city where an estimated one million people still live under the control of ISIL, also known as ISIS.
"Now that the formal announcement has been made, we are expecting the US-led coalition to carry out air strikes in support of what is expected to be a ground advance from a number of front lines around the city," she said.
But the launch of the operation marks only the start of a battle that is likely to be the most difficult in the war against ISIL.
"This could be a very long fight, or ISIL could choose to withdraw, it is very hard to say. But it is a complex battleground and a complex operation," our correspondent said.
According to UN estimates, up to one million people could be displaced from Mosul during the operation, exacerbating the humanitarian situation in the country.
"This is urban territory. There’s going to be street-to-street fighting. It’s densely populated and there is concern about the civilians who are basically trapped inside the city," Al Jazeera’s Khodr said.
In the lead-up to the planned operation, Iraqi aircraft dropped "tens of thousands" of leaflets early on Sunday, some bearing safety instructions for Mosul residents, the military said.
Iraqi troops were also positioned east of Mosul in the Khazer area, along with Kurdish Peshmerga forces, and to the north of the city near the Mosul Dam and Bashiqa areas.
Before Abadi’s announcement, Brigadier-General Haider Fadhil told the Associated Press news agency in an interview that more than 25,000 troops, including paramilitary forces made up of Sunni tribal fighters and Shia militias, would take part in the offensive that will be launched from five directions around the city.
Our correspondent said: "It’s a very uneasy alliance. Let’s take the Peshmerga and the Iraqi army, for example. Yes, the US has mediated between the central government in Baghdad and the Kurdish regional government and they resolved regional disputes, but there’s also the issue of disputed territories."
The alliance of Iraqi forces fighting to retake Mosul include the Iraqi army, the Kurdish Peshmerga and the Popular Mobilisation Forces, shia militia groups that now have official status from Baghdad.
For the past two years, the Iraqi army has struggled to regain control over vast parts of the country that fell to ISIL - even struggling to maintain security in the capital.
Baghdad still suffers frequent explosions, car bombs and suicide attacks, mainly in crowded areas.
The ongoing fighting between Iraqi forces and ISIL has displaced more than three million Iraqis and left an estimated 10 million in need of humanitarian assistance, according to the UN.
The UN has described the country’s crisis as "one of the world’s worst".