"Humanitarian agencies are rushing to provide assistance to more than 90,000 people fleeing clashes in Anbar governorate," the UN said in a statement
"Our top priority is delivering life-saving assistance to people who are fleeing -- food, water and shelter are highest on the list of priorities," it quoted Lise Grande, humanitarian coordinator for the UN in Iraq, as saying.
At least 2.7 million people have been displaced in Iraq since the beginning of 2014, including almost half a million from the western province of Anbar, the UN said.
Signup to our newsletter and follow us on Facebook and Twitter!
IS spearheaded an offensive last June that overran large areas north and west of Baghdad, including swathes of Anbar.
Anbar is a vast desert province that stretches east from the Baghdad governorate to the western borders with Saudi Arabia, Jordan and Syria.
Parts of Ramadi and all of the city of Fallujah, to its east have been out of government control for since early 2014.
Iraqi security forces performed dismally in the early days of the IS drive, but have regained significant territory with backing from mainly Shiite paramilitaries, a US-led coalition and Iran.