Heavy fighting between Syrian rebels and troops sent thousands fleeing across the border into Turkey on Friday, AFP reported.
The clashes over a border post near the northeastern town of Ras al-Ain killed 46 combatants in two days, a watchdog said, and highlighted the humanitarian crisis and regional dangers posed by Syria's nearly 20-month conflict.
The United Nations said more than 11,000 Syrians had fled into neighboring countries in the previous 24 hours alone, including 9,000 into Turkey, bringing to more than 408,000 the number of registered Syrian refugees in the region.
The UN said the total number of Syrian refugees in Turkey, Lebanon, Jordan and Iraq would likely hit 700,000, and the head of its humanitarian efforts said those in need of emergency aid in Syria would rise to more than four million early next year.
"This will just continue to grow in the terms of humanitarian suffering," John Ging, who heads the UN's Office for the Coordination of Humanitarian Affairs (OCHA), told reporters in Geneva, according to AFP.
At least 20 Syrian soldiers were killed in Friday's clashes over the Ras al-Ain post, one of only two crossings on the Turkish border still in regime hands, the Syrian Observatory for Human Rights said.
It said rebels had captured numerous soldiers and taken control of key positions in mainly Kurdish Ras al-Ain, including the offices of the security and intelligence services.
Turkish officials said 8,000 refugees had fled the fighting in Ras al-Ain overnight and that most were taken to a camp in Akcakale near the border.
In an interview with Russian television, excerpts of which were released Thursday, Syrian President Bashar al-Assad made it clear that he would not go quietly and warned that any foreign attempt at military intervention would be dangerously costly to the West.
"I think that the cost of a foreign invasion of Syria -- if it happens -- would be bigger than the entire world can bear... This will have a domino effect that will affect the world from the Atlantic to the Pacific," he told Russia Today.
"I do not believe the West is heading in this direction, but if they do, nobody can tell what will happen afterwards," he said.
Assad warned Syria was facing a protracted conflict because foreign powers were backing rebels fighting his regime, but insisted there was no civil war.
He admitted divisions existed in the country, but said "division does not mean civil war," and denied his forces had committed war crimes.
"I am not a puppet and the West did not make me in order that I leave to the West or any other country. I am Syrian, I am Syrian-made, and I must live and die in Syria," he said.
On Friday at least 114 people were killed nationwide, AFP reported, including 53 soldiers and 47 civilians.
Thousands of protesters rallied on Friday, with many mocking Assad's "live and die" remarks.
"Bashar, you will die in Syria, but you won't be buried in the ground, you will be thrown in the dustbin of history!" read one sign held by protesters in the central city of Hama.