Iraqi prime minister Haider al-Abadi has announced the start of operations to liberate the northern city of Mosul from Islamic State (IS) militants.
Mosul is Iraq's second largest city and the push to retake it will be the biggest military operation in Iraq since American troops left in 2011. If successful, it will be the biggest blow yet to IS.
The Iraqi prime minister said: "These forces that are liberating you today, they have one goal in Mosul, which is to get rid of Daesh (IS) and to secure your dignity. They are there for your sake.
"God willing, we shall win."
A statement on Mr Al-Abadi's website said the fight for the city marked a new phase that would lead to the liberation of all Iraqi territory from the militants this year.
Iraqi forces have been massing around the city in recent days. They include members of the elite special forces, who are expected to lead the charge into the city itself.
Mosul is home to more than a million civilians. The city fell to IS fighters in June 2014, leaving nearly a third of Iraq in militants' hands and plunging the country into its most severe crisis since the US-led invasion in 2003.
After seizing Mosul, IS leader Abu Bakr al-Baghdadi visited the city to declare an Islamic caliphate which at one point covered nearly a third of Iraq and Syria.
Since late last year, the militants have suffered battlefield losses in Iraq and their power in the country has largely shrunk to Mosul and small towns in the country's north and west.
Mosul is about 225 miles north-west of the capital, Baghdad.
The operation to retake Mosul is expected to be the most complex yet for Iraq's military, which has been rebuilding from its humiliating 2014 defeat.
Iraqi forces began moving into Nineveh province to surround Mosul in July, when ground troops led by the country's elite special forces retook Qayara air base south of the city.
Thousands of Iraqi troops were deployed there ahead of the planned operation along with large numbers of tanks, armoured personnel carriers and heavy artillery.
The base is ringed by a series of trenches, sand berms and other fortifications.
Iraqi troops are 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 the prime minister's announcement, Brig Gen Haider Fadhil said more than 25,000 troops, including paramilitary forces made up of Sunni tribal fighters and Shia militias, will take part in the offensive that will be launched from five directions around the city.
In addition to carrying out air strikes, the US-led international coalition will also offer artillery fire. American troops are providing logistical support to the operation.
Brett McGurk, the US envoy to the coalition against IS, said Mr Abadi's order called for "major operations" to free the city.
"Godspeed to the heroic Iraqi forces, Kurdish #Peshmerga, and #Ninewa volunteers. We are proud to stand with you in this historic operation," he tweeted.
The role of the Shia militias has been particularly sensitive, as Nineveh is a majority Sunni province and Shia militia forces have been accused of carrying out abuses against civilians in other operations in majority Sunni parts of Iraq.
According to UN estimates, up to one million people could be displaced from Mosul during the operation, exacerbating the humanitarian situation in the country.
The conflict in Iraq has forced 3.3 million people to flee their homes, with most of them living in camps or informal settlements.
Brig Gen Fadhil voiced concern about potential action from Turkish troops based in the region of Bashiqa, northeast of Mosul. Turkey sent troops to the area late last year to train anti-IS fighters there.
But Baghdad has seen the Turkish presence as a "blatant violation" of Iraqi sovereignty and has demanded the Turkish troops withdraw, a call Ankara has ignored.