Iraqi forces begun their operation to retake the Islamic State stronghold of Mosul, Prime Minister Haider al-Abadi announced.

“The time of victory has come and operations to liberate Mosul have started,” Abadi said, in an address broadcast on state television. “Today I declare the start of these victorious operations to free you from the violence and terrorism of Daesh [Islamic State, formerly ISIS/ISIL].”

READ MORE: Offensive on ISIS ‘capital’ Mosul could start Monday, Red Cross warns of 1mn refugees

An offensive to retake Iraq’s second largest city has been planned for months, since Iraqi forces and loyal government militia surrounded Mosul, in the country's north. The city fell to Islamic State (IS, formerly ISIS/ISIL) in June 2014, when the terrorists conducted their offensive on Iraq, overrunning nearly a third of the country.

Before the offensive began, dozens of ambulances were lined up at the frontline ready to ferry out casualties, as thousands of Iraqi troops moved into battle positions, The Washington Post reported.

Ahead of the offensive, the country's air force allegedly dropped leaflets warning the residents of the IS stronghold of the looming US-supported offensive. Iraqi forces have also cut off any escape routes for ISIS fighters.

Not all people in #Mosul are #ISIS members , be careful where you targeting , alot of wounded between civilians pic.twitter.com/GpjHZ4yDeS — Sound and Picture (@soundandpic) October 17, 2016

A day before the offensive, the US Special Presidential Envoy for the Global Coalition to Counter ISIL, Brett McGurk, warned of the unpredictability of the Mosul battle.

"This will be a very unpredictable, very dynamic, very uncertain operation," McGurk said, NBC reported. "We do not know what Daesh is going to do in Mosul."

Just ahead of the operation, the International Committee of the Red Cross (ICRC), warned of the humanitarian catastrophe that the offense could cause, saying that the battle could create a million refugees.

Godspeed to the heroic Iraqi forces, Kurdish #Peshmerga, and #Ninewa volunteers. We are proud to stand with you in this historic operation. — Brett McGurk (@brett_mcgurk) October 16, 2016

Some 30,000 Iraqi and Kurdish forces are now up against an estimated 3,000 to 4,500 IS militants in Mosul, where approximately 1 million civilians are now caught in the crosshairs.

UN emergency relief coordinator Stephen O’Brien issued a statement expressing his concern for the civilian population in Mosul.

“I am extremely concerned for the safety of up to 1.5 million people living in Mosul who may be impacted by military operations to retake the city from ISIL. Families are at extreme risk of being caught in cross-fire or targeted by snipers. Tens of thousands of Iraqi girls, boys, women and men may be under siege or held as human shields,” O’Brian said in a statement.

READ MORE: US, Saudis to grant 9,000 ISIS fighters free passage from Iraqi Mosul to Syria – source

The UN fears that thousands may be forcibly trapped between the fighting lines, warning that “children, women, the elderly, and disabled will be particularly vulnerable.”

“Depending on the intensity and scope of the fighting, as many as one million people may be forced to flee their homes in a worst-case scenario,” the UN official stressed, calling on the warring parties to respect international law and spare civilian lives.

In addition to Iraqi and militia forces, it is widely believed that US troops could also participate in the offensive. Late last month, President Obama authorized the dispatch of an additional 600 American troops to Iraq to assist with the Mosul operation. Before the announcement, the US had 4,500 troops in Iraq, years after the Obama administration officially withdrew all American troops from the country.

Meanwhile, US Defense Chief Ash Carter has welcomed the offensive on Mosul, calling it a "decisive moment” in the campaign to defeat Islamic State.

“We are confident our Iraqi partners will prevail against our common enemy and free Mosul and the rest of Iraq from ISIL's hatred and brutality,” he said in a statement. “The United States and the rest of the international coalition stand ready to support Iraqi Security Forces, Peshmerga fighters and the people of Iraq in the difficult fight ahead.”