The UNHCR said the Iraqi offensive could result in the displacement of one million people
The United Nations warned on Tuesday that a military offensive to retake the Iraqi city of Mosul from the Islamic State group could result in the worst population displacement in years.
The office of the UN High Commissioner for Refugees said that 200,000 Iraqis had already been forced to flee their homes by conflict since March this year.
Iraqi forces are currently involved in shaping operations aimed at tightening the noose on Mosul and setting the stage for a major assault on the jihadists' last major stronghold in the country.
The UNHCR said such an offensive could result in the displacement of an additional one million people.
"Worse is yet to come", the UNHCR representative in Iraq, Bruno Geddo, said. "We predict that it could result in massive displacement on a scale not seen globally in many years."
Nearly 3.4 million people have already been displaced in Iraq since the start of 2014.
Mosul is Iraq's second city and had an estimated population of around two million before IS took it over in June 2014 in an offensive that sparked large-scale displacement.
Accurate numbers for the population remaining there are hard to come by but the UN and other officials have said that up to one million civilians may still be living under IS rule in the Mosul area.
"We are building new camps and pre-positioning emergency relief kits to ensure people fleeing get rapid assistance," Geddo said.
"But even with the best-laid plans, there will be insufficient camps for all families needing shelter and we need to prepare other options," he added.
Iraqi special forces on Tuesday launched an operation to retake Qayyarah, a town on the banks of the Tigris river about 60 kilometres (35 miles) south of Mosul.
The town is expected to become a key launch pad for a broader offensive on Mosul in the coming weeks or months.
The Iraqi authorities and the aid community, including the UN, came under criticism for failing to cope with the much smaller influx of displaced people triggered by the successful operation to retake the city of Fallujah in June.