iStock/Thinkstock(MOSUL, Iraq) — As many as 1.5 million Iraqis are bracing for a major offensive on the city of Mosul, an operation that could force as many as 1 million people from their homes and send as many as 100,000 refugees across the border into war-ravaged Syria.

“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,” said Stephen O’Brien, United Nations Under-Secretary-General for Humanitarian Affairs and Emergency Relief Coordinator.

“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.”

Some 4 million Iraqis have been displaced and more than 24,000 killed since 2014, when the Islamic State, sometimes known as ISIL, tore through the country, capturing numerous key population centers, including Mosul, the country’s second-largest city. This amounts to one out of every ten Iraqis who have been forced to flee their homes in a fresh convulsion of violence as the U.S. marks five years since the end of its combat operations in the country.

Aid groups have been rushing to prepare for what could be the largest and most complex humanitarian operation in a year that has already seen several major disasters, the UN said on Sunday. As many as 200,000 could flee during the first weeks of fighting and as many as 700,000 could require emergency shelter.

“We are mobilizing significant resources to set up camps to accommodate displaced people in safety, and to provide relief items to families who may be on the move. There is uncertainty around the military situation,” said Filippo Grandi, the UN High Commissioner for Refugees, as he began a four-day tour of the country on Saturday.

The rise of ISIL in Iraq has set off a humanitarian catastrophe in a country still struggling to rebuild after the U.S.-led invasion in 2003 that led to more than eight years of war and ongoing sectarian conflict.

The number of people who need some form of humanitarian assistance has doubled since last year and now exceeds 10 million, nearly a third of Iraq’s population, the UN says. Of that total, over 8.5 million require access to essential healthcare, 6.6 million people need critical services such as water and sanitation, and at least 2.4 million are in need of food.

A combined force numbering 30,000 Iraqi and Kurdish forces began encircling Mosul on Monday, after months of planning and preparation. Lt. Gen. Stephen Townsend, commander of Combined Joint Task Force Operation Inherent Resolve, said the operation to take Mosul from ISIL could take weeks, or possibly longer.

The general cautioned that it “may prove to be a long and tough battle.” The U.S. is assisting the offensive with air support, artillery, intelligence, advisors and forward air controllers, Gen. Townsend said.

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