Aid groups warn huge numbers of civilians are at risk as a major offensive to recapture ISIL-held Mosul gathers pace.

As Iraqi armed forces and their allies move to reclaim the city of Mosul from ISIL fighters, humanitarian agencies are warning that the lives of a huge number of civilians are in grave danger.

Hours after the launch of the massive ground and air operation early on Monday, the United Nations said an estimated 1.5 million people still live in the Mosul area, and up to one million people could flee their homes in search of safety.

Such a movement would create the biggest and most complex humanitarian crisis in the world this year, amid a wide-scale anti-ISIL operation involving Iraqi forces, Kurdish Peshmerga fighters, allied militias and US-led coalition air strikes.

“As the Mosul offensive begins, protecting civilians must be an essential part of military strategies,” said Filippo Grandi, UN high commissioner for refugees.

“This is crucial for the future of Iraq.”

‘Enormous danger’

Mosul, Iraq’s second biggest city, is the last urban centre still under the armed group’s control in the country.

Analysts expect the battle for Mosul to last several weeks, and even months, exacerbating an already dire situation for residents struggling to afford food and access medicine.

IN PHOTOS: Knocking on Mosul’s door

“Civilians in Mosul are faced with an impossible choice and enormous danger whether they stay or flee,” according to Alun McDonald, a spokesman for international charity Save the Children who is based in Duhok, north of Mosul.

“If they stay put inside the city, they risk being killed in the crossfire or bombed, and trapped without aid,” McDonald told Al Jazeera.

“If they try to flee, they risk being killed by snipers or landmines planted around the city.”

But even if they do manage to make it out, there are only seven emergency camps set up outside Mosul with only 60,000 tents available, whereas hundreds of thousands are expected to flee as the fighting intensifies.

“We’re in a race against time to ensure that enough displacement camps are set up to receive the people who might flee Mosul in the coming days,” Karl Schembri, an Iraq-based spokesman for the Norwegian Refugee Council (NRC), told Al Jazeera.

Peshmerga forces in the east of Mosul as they prepare to attack ISIL positions [Azad Lashkari/Reuters]

‘Safe routes’

In the lead-up to the long-awaited operation, Iraqi jets dropped “tens of thousands” of leaflets on Mosul, some bearing safety instructions for residents, including asking families to stay indoors and put white flags on their homes.

But aid groups say such measures are not going to be enough to protect vulnerable parts of the population.

The priority, they insist, is to establish truly safe routes out of the city – despite a number of security challenges, including the potential presence of landmines and booby traps around Mosul.

“Ensuring there are safe corridors is a matter of life or death for civilians,” McDonald said.

“We’ve seen in Aleppo [in Syria] at the moment the terrible impacts on children of being trapped in a military battlefield without any way to escape, so we need to make sure the same thing doesn’t happen in Mosul.”

Schembri said safe corridors are essential to get people evacuated securely, but added that their establisment entails “negotiation and agreement between all parties to the conflict, which is unlikely.

“It would mean agreeing on times and places in which none of the warring parties would attack civilians as they head out of the city, give them protection, while allowing aid and humanitarian assistance in to the people who need it most.”

‘Moment of truth’

The battle to recapture Mosul comes after a series of Iraqi government offensives to reverse ISIL’s territorial gains since 2014, including in Fallujah, Ramadi and elsewhere.

Upon announcing the launch of the offensive, Iraqi Prime Minister Haider al-Abadi addressed residents in Mosul, a prominently Sunni city, saying that the goal of the advancing forces was to “get rid” of ISIL and “secure your dignity”.

“Very soon, we will be among you to raise the Iraqi flag … we have come to rescue you and save you from terror,” he said.

Following a meeting with Abadi on Monday, Grandi, the UN refugee chief, said that he had received assurances from the government that civilian protection would be part of their military strategy to retake the city.

Grandi also said that the security screening of those fleeing Mosul should be conducted “in the most appropriate manner”.

Back in June, some Iraqis who had escaped ISIL-held Fallujah said they were tortured while held captive by government-allied sectarian militias.

In July, Zeid Ra’ad al Hussein, the UN human rights chief, said there were “extremely distressing, credible reports” that civilians fleeing the fighting in Fallujah were facing extreme abuse and even death at the hands of Shia armed groups allied with the government troops.

“The way this offensive is carried out, as well as the treatment of the fleeing civilians at the hands of the fighting forces, and the effectiveness of the humanitarian response, will determine the future of Iraq and how Iraqis will live side by side with each other,” Wolfgang Gressmann, country director for the NRC, said.

“We cannot let Iraqi civilians down again in this moment of truth.”