Most of the displaced in Iraq were forced to flee their homes during successive waves of violence in early 2014 and after jihadist offensives in June and August.
Violence continues to force thousands from their homes, and the United Nations Assistance Mission in Iraq said in an update that one of the most affected areas in recent days had been the Kirkuk region.
"More humanitarian actors and greater attention are urgently needed in order to expand the provision of assistance to IDPs (internally displaced people) in Kirkuk," the UN said.
It said snow had begun falling on parts of the Dohuk governorate, the northwestern Kurdish province which hosts the largest number of IDPs in Iraq.
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"The lack of proper shelter, thermal insulation of shelters and non-food items will eventually negatively impact the health status of displaced populations during winter," the UN report said.
The Action Against Hunger (ACF) aid organisation said displaced people often live in precarious conditions in open spaces, camps and unfinished buildings.
"If the winterisation response is not scaled up immediately, we will see both short and long term impacts on the IDP population," the group said in a statement.
The funding gap for this programme alone in the huge crisis response effort needed to assist the millions affected by war in Syria and Iraq stands at $173 million.