GENEVA — The United Nations said on Tuesday that it was bracing to cope with hundreds of thousands of civilians likely to flee their homes as Iraqi security forces begin operations to drive Islamic State forces from their last remaining strongholds in Iraq.

Iraqi forces recaptured Mosul in July after nine months of fighting that caused the evacuation of more than 950,000 residents and left large swaths of the western half of the city in ruins. Lise Grande, the United Nations coordinator in Iraq, said the Iraqis were poised to begin operations to recapture the towns of Tal Afar and Hawija, as well as the Euphrates valley in western Anbar province.

“We think that by the end of those operations, several hundred thousand more civilians are likely to be displaced,” Ms. Grande told reporters in Geneva, adding a new layer of deprivation to one of the world’s biggest humanitarian crises.

Hundreds of thousands of Mosul residents have already returned to the city, she said, but more than 3.3 million Iraqis remain displaced from their homes by conflict across the nation. Aid agencies expect that figure to increase to about 3.5 million as military operations unfold in the coming months, she said.