Up to 1.2 million people in and around Mosul could be affected by an assault to retake the northern Iraqi city from so-called Islamic State, the UN says.
A spokesman for the UN Refugee Agency said the humanitarian impact of the planned offensive by pro-government forces was expected to be “enormous”.
More than 120,000 people have been displaced by fighting since March as troops clear territory to the south.
Earlier, they launched an operation to recapture the key town of Qayyar
A spokesman for the Counter-Terrorism Service (CTS), Sabah Numan, told the BBC that its elite forces were battling IS militants on the outskirts.
Mr Numan said they were working in co-ordination with armed residents inside.
Pro-government forces have preparing to move on the town for weeks, after recapturing a nearby airbase that will be used as a logistics hub for the wider offensive on Mosul, 60km (40 miles) to the north.
The mayor of Qayyarah, Saleh al-Jubouri, said an estimated 15,000 civilians were trapped in the town, but that most of the militants had fled or been killed.
More than 3.38 million people have been forced to flee their homes in Iraq since 2014, when IS militants overran large parts of the country’s north and west and proclaimed the creation of a caliphate stretching into neighbouring Syria.
An additional one million Iraqis were displaced as a result of the sectarian conflict that followed the US-led invasion in 2003.
“Worse is yet to come,” warned the representative of the UN High Commissioner for Refugees (UNHCR) in Iraq, Bruno Geddo. “We predict that it could result in massive displacement on a scale not seen globally in many years.”
Once the battle for Mosul begins, a further 400,000 people are expected to escape to the south of the city, 250,000 to the east and another 100,000 to the north-west.
UNHCR spokesman Adrian Edwards told reporters in Geneva that contingency plans had been drawn up to provide shelter for up to 120,000 people fleeing Mosul, while the agency was looking to set up six new camps across northern Iraq. -BBC