As many as 11,000 people have fled Syria in 24 hours - one of the biggest refugee exoduses the country has seen in its 20-month conflict.
The refugees were escaping fierce fighting between rebels and government forces for control of the northeastern Syrian town of Ras al Ain on the border with Turkey.
The exodus signals the escalating ferocity of the conflict, which has killed more than 36,000 people since March 2011.
The United Nations has warned that an estimated four million people inside Syria will need humanitarian assistance by early next year as winter sets in - up from 2.5 million now.
Of the 11,000 Syrians who fled in the 24-hour period that began on Thursday, 9,000 crossed into Turkey, while Jordan and Lebanon each absorbed another 1,000 refugees, according to UN officials.
Video from Turkey's news agency Anadolu showed Syrians jumping over and climbing through a razor-wire fence on the border to cross into the Turkish town of Ceylanpinar.
The influx has caused alarm in Turkey, which has long expressed worry over its ability to cope with such large numbers and has called for a buffer zone to be set up inside Syria where refugees could be housed.
Turkish soldiers guard the border with Syria near the town of Ceylanpinar The flood of Syrians into Turkey, Jordan and Lebanon was "the highest that we have had in quite some time," said Panos Moumtzis, the UN refugee agency's coordinator for the region.
Despite the bloodshed, President Bashar al Assad said in a rare TV appearance that there was no civil war in Syria.
"It is about terrorism and the support coming from abroad to terrorists to destabilise Syria. This is our war," Mr Assad said in an interview by broadcaster Russia Today, which was aired on Friday.
Mr Assad has insisted he would not step down, saying he would "live and die in Syria".
But Syrians still in the country faced an increasingly desperate situation, senior UN official John Ging, in Geneva, said.
"Every day our humanitarian colleagues on the ground are engaging with people who are ever more desperate, ever more fearful for their lives and for the lives of their families because of this conflict," he said.
Also on Friday, Syria's main opposition bloc in exile, the Syrian National Council, elected veteran activist George Sabra, a Christian, as its new head.
The group has come under heavy criticism from international allies for being ineffective in the fight against Mr Assad and for being riven by personal disputes.
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