Some 66,000 refugees - mainly Syrian Kurds - have crossed into Turkey in 24 hours, officials say, as Islamic State militants advancein northern Syria.
Turkey opened its border on Friday to Syrians fleeing the Kurdish town of Kobane in fear of an IS attack.
Activists say some 300 Kurdish fighters have crossed into Syria from Turkey to help defend the strategic town.
IS controls large areas of Syria and Iraq and has seized dozens of villages around Kobane, also called Ayn al-Arab.
Turkey - which shares a border with Iraq and Syria - has taken in more than 847,000 refugees since the uprising against Syrian President Bashar al-Assad began three years ago.
But the opening of the border has seen a dramatic increase in the past 24 hours.
"As of today, the number of Syrian Kurds who entered Turkey has exceeded 60,000," Turkish Deputy Prime Minister Numan Kurtulmus told reporters on Saturday.
He was speaking from the southern Turkish province of Sanliurfa, where many of the refugees have sought shelter.
Separately, a Turkish government official told the BBC's Mark Lowen that the number is as high as 66,000.