The UN refugee agency warned Tuesday that as many as 400,000 people may flee to Turkey from Syria's Kurdish region to escape attacks by the Islamic State group.
UNHCR spokeswoman Melissa Fleming told reporters that 138,000 terrified Syrians, mainly Kurds, had escaped to Turkey since Friday.
Kurdish militia in Syria are battling to defend the key northern border town of Ain al-Arab, or Kobane, from an IS onslaught.
IS has already seized dozens of villages and, amid reports of executions and other atrocities, residents have spilled into Turkey.
"We're preparing for the potential of the whole population fleeing into Turkey. Anything could happen. And that population of Kobane is 400,000," Fleming told reporters.
Of those, 200,000 had earlier fled from other war-ravaged regions of Syria to what had been a relative safe haven, she underlined.
The IS jihadists, who have also rolled through swathes of neighbouring Iraq over recent months, are trying to secure their grip over a long stretch of Syria's border with Turkey.
Turkey, already hosting some 1.5 million refugees from more than three years of war in Syria, has come under mounting pressure amid the latest influx.
Kurdish rebels in Turkey have called on their community to cross into Syria to fight IS, but Ankara is controlling its border tightly, sparking clashes with angry locals.
Fleming said Turkey told UNHCR it was now allowing entry via two border crossings, down from nine, and was conducting security checks to ensure the refugees were civilians.
"There is a war next door, very, very close, so obviously they have to take their own security precautions with their borders," said Fleming.
"That said, they have welcomed over a million and a half Syrian refugees. Obviously it's a big influx for any country to cope with," she added.
UNHCR repeatedly has called on rich nations to do more to help Turkey and Syria's other neighbours -- together hosting over three million refugees from the war which broke out in 2011.
Fleming said the latest influx threw the spotlight squarely on that issue.
"If you want to put it into proportion, this 138,000 that has just arrived in Turkey represents about the number that all of Europe has taken in in the three years of the Syria war," she said.
The impact of the IS offensive is also being felt in Jordan.
Fleming said that for the first time, refugees from northern Syria now make up the majority of the 250 people fleeing into Jordan every day.
IS attacks have also driven hundreds of thousands of Iraqis from their homes.
Fleming said that Iraqi arrivals in Jordan had risen sharply to 120 per day in August and September, four times the rate seen earlier in the year.
Inside Iraq, more than 1.7 million people have been displaced by violence, sparking a massive humanitarian crisis.
Elisabeth Byrs, spokeswoman for the UN's World Food Programme, said her agency was now supplying life-saving aid to one million of them.