Iraq has launched its biggest fight against Isis.

In a televised address, Prime Minister Haider al-Abadi pledged to raise the Iraqi flag over Mosul once more, calling on residents to cooperate with advancing forces.

The US-backed operation aims to push Isis (Islamic State) out of its de facto capital in Iraq. More than a million civilians are believed to be trapped in the city.

Dozens of ambulances were lined up at checkpoints on the edges of Iraq's northern region of Kurdistan, ready to ferry out casualties. Thousands of Iraqi troops have moved into position for the battle in recent weeks, and new military staging areas have sprung up along front lines.

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The Mosul offensive marks a showdown in Isis' last major stronghold in Iraq. It was in Mosul's Great Mosque that Isis leader Abu Bakr al-Baghdadi announced his self-proclaimed caliphate more than two years ago.

But since then, the group's grip has slowly crumbled. Tikrit, Ramadi and Fallujah have been clawed back by Iraqi forces, with a heavy reliance on US-led airstrikes.

"We will soon meet in Mosul to celebrate in liberation and your salvation," Abadi said, addressing the people of the city. "We will rebuild what has been destroyed by this criminal gang."

The battle for Mosul draws together tens of thousands of Iraqi troops from an array of the country's forces: Kurdish peshmerga soldiers, Sunni tribal fighters, army, police, Shia militias and elite counterterrorism units.

From the sky and on the ground comes close support from the US-led coalition. More than 80,000 troops are involved, including engineers and logistical support, said Major Salam Jassim, a commander with Iraq's elite special forces.

Despite sometimes competing agendas, the various armed forces have united, at least for now. At a staging area in a hamlet near Khazir, east of Mosul, Jassim and his men were waiting for the order for "zero hour". Battle plans were drawn out in black marker on walls and plastic tables. "We'll take it," Jassim said. "There's no doubt."

A Peshmerga convoy drives towards a frontline in Khazer, about 30 kilometers (19 miles) east of Mosul, Iraq. Photo / AP
A Peshmerga convoy drives towards a frontline in Khazer, about 30 kilometers (19 miles) east of Mosul, Iraq. Photo / AP

Troops have massed to the north, south and east of the city. Trucks packed with Iraqi soldiers and military vehicles have clogged the roads. Tanks, armoured vehicles and weaponry have been hauled nearly 400km from the capital, Baghdad.

For the first 48 hours, the offensive on the eastern front will be led by Kurdish peshmerga, Iraqi military officers said.

Brigadier General Haider Obaidi, another commander with Iraq's special forces, said: "We'll start after them and move after them to support them."

Police and Iraqi Army units will move up the main highway from Baghdad, while Shia militia forces are expected to focus on Tal Afar to the west and the town of Hawijah to the southeast. Kurdish peshmerga forces, Sunni fighters and the Iraqi army will also attack from the north.

Opinions are split on just how long and grinding the battle will be. Abadi has pledged to have the city back under Iraqi control by the end of the year. Jassim is not sure that's possible, with booby traps and explosive devices expected to slow the way. Civilians will complicate the battle. Between 1.2 million and 1.8 million are still inside the city, he said.

Prime Minister Haider al-Abadi announces the start of the operation to liberate the northern city of Mosul from Islamic State militants. Photo / AP
Prime Minister Haider al-Abadi announces the start of the operation to liberate the northern city of Mosul from Islamic State militants. Photo / AP

To avoid a humanitarian crisis, the Iraqi Government has asked civilians to stay in their homes, complicating air support and operations to clear neighbourhoods of militants. "The operation will take much longer because of this," Obaidi said. "For their safety, but it also means each neighbourhood needs to be surrounded and searched as we clear it."

Still, the US-led coalition will give closer support than in any other operation, he said, and Apache helicopters will probably be used. The coalition has requested that the airspace be cleared of Iraqi jets, whose air support will be limited to the areas where Shia militias are on the ground.

The western side of the city will be left largely open, which may make for a less protracted fight inside than if it was besieged. "We'll try to give them an escape to run to Syria," he said of the militants.

Brigadier General Yahya Rasoul, a spokesman for the Iraqi military, said that even if the western side is left open, it doesn't mean a safe escape for Isis. "If we do that, then this area will become a killing zone as we target them with our aircraft," he said.