The number of people fleeing the war in eastern Ukraine to other parts of the country has jumped from 2,600 to 102,600 inside two months, the UN says.
Its figures for early June to early August coincide with a sharp increase in fighting between pro-Russian separatist rebels and security forces.
More than 1,000 people are leaving the combat zone every day, the UN says.
A further 168,000 have crossed into neighbouring Russia, which has been accused of fuelling the insurrection.
At the scene: Tom Burridge, BBC News, Kiev
Inside the warehouse are several long lines of cramped bunk beds, made out of cheap chipboard. "It's intense living with so many other people," says Andrew, who has also been living on this industrial estate for two months with his wife Lina, and their four children. "You can't relax like you would at home."
The couple don't want to give their surname, because they fear about their future in eastern Ukraine, once the war there is over.
"At least we have a roof here and there are no bombs falling from the sky," says Lina. But although their three youngest children, aged four, five and nine, first saw their new warehouse home as an adventure, the novelty has worn off. "The children cry," Lina says. "They want to go home."
The UN says men of fighting age are often prevented from leaving rebel-controlled territory by the rebels.
It warns that some of the shelters in areas near the conflict zone are "primitive" and says there are shortages of basic things such as mattresses, bed linen and hygiene items.
At least 1,500 people, both civilians and combatants, are believed to have been killed and thousands more injured since Ukraine's new government sent in troops to put down the insurrection in the Donetsk and Luhansk regions in mid-April.
The rebels have since been pushed back to their strongholds in the two cities of the same name, though other pockets of resistance remain.
Fear of cross-fire
Before the current crisis erupted, Donetsk and Luhansk had a combined population of about 6.5 million people, or about 14% of Ukraine's overall population.
According to the UN, more than 117,000 people have been displaced by the turmoil in Ukraine since Russia annexed Crimea in March, while of these 85% are from the eastern regions.
Asked why they had fled the region, people gave several reasons to the UN
- security concerns, including the risk of being caught in crossfire
- fears of persecution for political views or ethnicity
- fears of forcible recruitment by government or anti-government forces
- fears of abduction, extortion and harassment
- damage to homes, lack of services and damage to basic infrastructure
For those left behind, the UN said, "basic services and infrastructure have been heavily affected by the increased violence, with scarcity of drinking water now becoming increasingly common.
"Many houses and buildings have been partially or totally destroyed in the areas affected by conflict in Donetsk and Luhansk regions."
Reliable numbers for casualties in Donetsk and Luhansk are difficult to establish, but a UN report last month recorded at least 522 civilian deaths and 258 military deaths. Scores more have been killed since then.
The rebels say they have lost at least 800 of their own fighters and claim the government is concealing the true scale of its military losses.
Meanwhile, in one of the most extraordinary episodes of the conflict, the fate of more than 300 Ukrainian soldiers who sought shelter inside Russia after being cut off by the rebels remains unclear.
The soldiers and border guards, who entered Russian territory on Monday, have been housed in tents supplied by the Russian border service while negotiations continue about their fate.
Russia says that 438 Ukrainian service personnel sought shelter near the town of Gukovo, in Russia's Rostov region, while the Ukraine government says there are 311 of them.
Media comment on Ukrainian soldiers in Russia: BBC Monitoring
"This is a new tactic employed by the Russians: push battle-worthy army units towards Russia and use television to create the impression that they have deserted and violated their oath" - Ukrainian journalist Andriy Tsapliyenko on Facebook
"As a human being, I can understand why they crossed into Russia. But I cannot justify them: they left those of their comrades-in-arms who chose to fight and break through. This cannot be justified" - Ukrainian journalist Yuriy Butusov on Facebook
"Many are sure: they were simply betrayed" - Russia's Channel One TV
"The mass desertion of National Guard fighters shows that confusion and chaos reign in the oligarch army" - Russian pro-government daily Izvestia
"We must save the lives of the people who have crossed the border and treat them as our Russian soul tells us: feed them, keep them warm and send them back to their mummies and daddies, if they wish so. In this way, we will show the grandeur of Russia" - State Duma defence committee chairman Vladimir Komoyedov, speaking to influential daily Kommersant
The BBC's Russian Service was told by the Ukrainians that they had been trapped by the rebels just inside the border at Krasnopartizansk, in Luhansk region. They had stopped receiving supplies by air after the crash of the Malaysia Airlines jet on 17 July, which is believed to have been shot down by a missile.
So fierce was the rebel bombardment, that the 72nd Mechanised Brigade lost a third of its men killed or injured in the course of a month, one sergeant said. In Soviet times, the brigade numbered some 3,000 soldiers.
Part of the brigade managed to break out of the encirclement and reach Ukrainian-held territory, a Ukraine official said.
The Ukrainians had also lacked adequate weapons against the rebels' artillery and tanks, the BBC Russian Service's Yury Vendik was told.
Plans were being made to return 180 of the soldiers to Ukraine, Russians security forces said.