The United Nations said on Tuesday it was rushing to build camps to accommodate what it expects to be a mass exodus from Mosul once a battle begins for Iraq's second city.
The UN refugee agency warned that an expected battle to liberate Mosul, the Islamic State group's last major urban stronghold in Iraq, was likely to "dramatically worsen" the displacement situation in the country.
Iraq is already facing one of the world's biggest displacement crises, with 3.38 million people forced to flee their homes in the country since 2014.
In just the past few months alone, 213,000 people have been forced from their homes across the country, including around 48,000 who have fled Mosul, UNHCR spokesman Adrian Edwards told reporters in Geneva.
"The humanitarian impact of a military offensive there is expected to be enormous," he said, cautioning that as many as 1.2 million civilians could be affected.
After retaking Fallujah, west of Baghdad, in June, the main focus of Iraqi security forces is Mosul, which is the IS group's de facto capital in Iraq.
Iraqi special forces led an operation Tuesday aimed at retaking the jihadist-held town of Qayyarah, which is expected to be used as a launchpad for a broader operation against Mosul in the coming weeks or months.
Once the operation begins, UNHCR estimates that some 400,000 people could flee to the south of Mosul, around 250,000 to the east and another 100,000 to the northwest, towards the Syrian border, Edwards said.
He said contingency plans had been drawn up to provide shelter for up to 120,000 people fleeing conflict in Mosul and surrounding areas, while UNHCR was looking to set up six new camps across northern Iraq.
"Progress depends on both the availability of land and of funding," Edwards said, pointing out that UNHCR's overall appeal for $584 million for displaced Iraqis, including those who have fled to nearby countries, was only 38 per cent funded.
He also warned that "finding available land for the new camps has become a critical issue", as many landowners were unwilling to lease land.
Other areas could not be used because they were too close to the frontline, or because setting up camps there could inflame ethnic, sectarian, religious or tribal tensions, he said.