More than 235,000 people have fled from their homes in Syria's rebel-held Idlib province, the UN says, as government forces step up their offensive on the area.
The exodus happened between 12 and 25 December as fighting intensified.
Idlib in north-western Syria is the last major region held by rebels opposed to President Bashar al-Assad.
The Russian-backed Syrian government has been bombarding Idlib since late November.
An increase in air strikes and ground fighting since mid-December has accelerated civilian displacement from the area, UN humanitarian agency Ocha said in a report.
Tens of thousands of families - including some already displaced numerous times during the civil war - have been heading north in trucks and private cars.
The government offensive had left the city of Maarat al-Numan and the nearby region in southern Idlib "almost empty", Ocha said.
Most of the displaced civilians are moving to cities and refugee camps in northern areas of Idlib and neighbouring Aleppo province.
"Many who fled are in urgent need of humanitarian support, particularly shelter, food, health, non-food and winterisation assistance," the UN said.
But the exodus has also been hampered by fuel shortages and the fact that some drivers don't want to risk being bombed as they drive.
Thousands more families were "frightened to move, fearing the risk of air strikes and shelling along the routes", the UN said.
A ceasefire negotiated by Russia, whose military campaign in support of Mr Assad has turned the tide of the eight-year civil war in his favour, and Turkey, which backs the opposition, halted a government assault on Idlib in August.
But the fighting has continued sporadically as President Assad attempts to wrest back control of Syria from rebel fighters and jihadists.
In September, the UN Office for the High Commissioner for Human Rights (OHCHR) said it had verified more than 1,000 civilian casualties in north-western Syria since April as a result of hostilities between pro-government and opposition forces.
On Thursday US President Donald Trump urged Russia, Syria and Iran to stop the violence in Idlib.
Russia, Syria, and Iran are killing, or on their way to killing, thousands of innocent civilians in Idlib Province. Don’t do it! Turkey is working hard to stop this carnage.— Donald J. Trump (@realDonaldTrump) December 26, 2019