As the battle to retake Mosul nears, human rights organizations, chiefly the U.N. refugee agency UNCHR, fear further displacement of Iraqis.

Hundreds of thousands in and around Mosul could be uprooted by the military assault to retake the city from ISIS, UNHCR said on Tuesday.

“In Mosul we believe the displacement situation may be about to dramatically worsen,” UNHCR spokesman Adrian Edwards told a briefing in Geneva, saying the agency needed more land for camps.

About 3.4 million people have already been forced by conflict to leave their homes across Iraq, taking refuge in areas controlled by the government or in the Kurdish autonomous region east of Mosul, ISIS’ de facto capital.

With a population at one time as large as 2 million, Mosul is the largest city under the ultra-hardline militants’ control in either Iraq or neighboring Syria.

Iraqi and Kurdish forces are gradually closing in on the city 400 km north of Baghdad, with air and ground support from a U.S.-led coalition.

Its fall would mark the defeat of ISIS in Iraq, according to Prime Minister Haider al-Abadi.

“Mosul will be liberated in 2016. We have a plan to liberate Nineveh,” he told a news conference on Tuesday in Baghdad. Nineveh is the province where Mosul is located and is home to a number of religious and ethnic groups including Christians, Turkmen and Kurds, in addition to Sunni Muslim Arabs.

Abadi said the government was in contact with all sides to find a way to manage Mosul, which he said would be done in cooperation between the federal government, the Kurdistan government, local residents and security forces.

The Iraqi army said on Tuesday reported clashes with militants in the town of Qayyara, near an air base it plans to use as a main center to support the Mosul offensive. The army captured the airfield last month and U.S. forces are helping to rehabilitate it.