More than 90,000 people have fled their homes in Iraq's western province of Anbar where Islamic State militants have been gaining ground over the past week, the United Nations said on Sunday.

Islamic State militants have encroached on the provincial capital Ramadi, displacing thousands of families.

"Our top priority is delivering life-saving assistance to people who are fleeing -- food, water and shelter are highest on the list of priorities," Lise Grande, humanitarian coordinator for the United Nations in Iraq, said in a statement.

Iraqi forces are preparing to mount a counter-offensive to reverse Islamic State advances on the eastern edge of Ramadi after military reinforcements were sent from Baghdad, officials said. Provincial officials warned earlier this week Ramadi was in danger of falling to the militants.

At least 2.7 million Iraqis have been displaced across the country since January 2014, including 400,000 from Anbar.