The United Nations says 235,000 civilians have fled their homes in rebel-held northwestern Syria during a Russian-backed campaign of airstrikes and shelling this month.
The UN Office for the Coordination of Humanitarian Affairs says the exodus took place between 12-25 December.
Most people had fled the city of Maarat al-Numan, towns and villages in southern Idlib province, Idlib city, and camps along the Syrian-Turkish border, OCHA said.
"With the latest escalation of violence in northwest Syria, civilians in Idlib governorate are again suffering from the devastating consequences of hostilities," it said.
OCHA said Maarat al-Numan and the surrounding countryside "are reportedly almost empty".
Syrian forces are advancing on Maarat al-Numan.
Thousands of families were also reported to be too frightened to move, fearing airstrikes and shelling.
Abu al-Majd Nasser, who fled to the border with his family from the town of Telmanas, said Russian President Vladimir Putin "wants to kill every Syrian who opposes the regime".
Moscow and Damascus deny claims of indiscriminate bombing of civilians, saying say they are fighting jihadist militants.
Civilians flee a Syrian military offensive in Idlib province, Syria. Source: AP
Rescue services and witnesses say hostilities have left many towns in ruins and knocked out dozens of medical centres.
US President Donald Trump has spoken out against the "carnage" involving thousands of civilians in Syria.
"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," Mr Trump said on Twitter on Thursday.
The Syrian army said this week it had seized more than 300km of territory in the assault to end "terrorist" control of Idlib, killing hundreds of "terrorists".
A Syrian opposition leader has called on the international community to help the millions of civilians in the country's last rebel-held stronghold, calling it a "disaster area".
Opposition leader Nasr Hariri told reporters in Istanbul the international community "should turn on the red lights because there is a humanitarian catastrophe inside Syria".
He added that large numbers of people are fleeing towards the Turkish border in what could trigger a new refugee crisis.
"We declare this area a disaster area and it should be dealt with accordingly," said Mr Hariri, who heads the High Negotiations Committee.
Donald Trump has called on Russia and Syria to stop "the carnage". Source: EPA
He said work should be done to reach a permanent ceasefire in Idlib, not a truce that would crumble later.
Turkish President Tayyip Erdogan said his country cannot handle a fresh wave of migrants from Syria, warning that Europe will feel the impact of such an influx if the bombing is not stopped.
