As many as 180,000 people have fled fighting between Iraqi forces and Islamic State (IS) militants in and around the city of Hit in western Anbar province, the UN says.
The civilians - many of whom were already displaced - have headed east towards the war-torn city of Ramadi.
The UN says the refugees are in need of food, blankets and medical supplies.
IS captured Hit earlier this month in an advance across Anbar that has alarmed Iraqi leaders.
Analysts say seizing Anbar would enable IS to establish a supply line to launch possible attacks on the Iraqi capital, Baghdad.
In other developments:
- At least 25 people are killed in a spate of bomb blasts in predominantly Shia areas of the capital, Baghdad
- UK Foreign Secretary Phillip Hammond, on his first official visit to Baghdad, says Iraqi forces will have to take the lead in fighting IS on the ground
- The US says Turkey has agreed to allow moderate Syrian rebels to be trained on its soil in its bid to combat IS
The US-led coalition that is battling IS has carried out air strikes on the area around Hit.
However, Iraqi security officials said on Monday that IS had overrun a military base the Iraqi army had abandoned about 8km (five miles) west of the city.
They took military vehicles, including tanks, and then set the camp ablaze, they said.
On Saturday, officials in Anbar appealed for military help, saying the province was in danger of falling to IS.
Anbar's provincial council asked the Iraqi government for US ground troops, Iraq's al-Sharqiyah TV reported.
Iraqi Prime Minister Haider al-Abadi has repeatedly ruled out any foreign ground troops in Iraq.
US Secretary of State John Kerry also said in Cairo at the weekend that Iraqis would have to do the fighting on the ground.
"Ultimately it is Iraqis who will have to take back Iraq," he said. "It is Iraqis in Anbar who will have to fight for Anbar."
The province is home to Iraq's second-largest dam, the Haditha, which controls the flow of the Euphrates to southern Iraq.
IS already controls large swathes of territory across Syria and Iraq.
Sold into slavery
"Reports indicate that 30,000 families, or approximately 180,000 individuals... many experiencing their second, third or fourth displacement, have left the city (Hit) in the past few days," a statement from the UN's Office for the Co-ordination of Humanitarian Affairs (OHCA) says.
"The displaced dispersed to Ramadi, Khaldiya, Hajaj and Amiryah Rahaliya. Many fled in vehicles taking basic household items and found refuge with other IDPs (internally displaced persons) in public buildings, such as schools."
Meanwhile, an article in IS's online magazine Daqib said that Yazidi families have been sold into slavery by the group.
The Yazidi people lived in an area of north-west Iraq overrun by IS fighters in August.
The article says the people were divided among IS fighters "according to Sharia law" and goes on to describe slavery as a type of pathway to Islam.
A recent Human Rights Watch report said IS had systematically separated captured women and girls from their families and forced some to marry its fighters.
Reports of atrocities have also come from the northern Syrian town of Kobane where IS fighters are battling Kurdish forces.
Witnesses have reported IS fighters beheading civilians trying to flee to safety.
Kobane official Feyza Abdi, who is now a refugee in Turkey, said IS had control of the town to the south, east and west, and warned of a "massacre" if they took the northern front bordering Turkey.
"That is what they want, to completely enclose the town, cut off all contact with Turkey and engage in barbarism," he said.
Kurdish forces say they urgently need more weapons and ammunition to push back the militants' advance in the town.
The Pentagon said on Monday that US and Saudi warplanes had carried out eight air strikes in the past 24 hours against IS positions around Kobane.