Turkey says more than 66000, mainly Kurdish, refugees from northern Syria have crossed the border in 24 hours since it was opened by the Turkish authorities.
The flow of refugees has been prompted by intense fighting between Islamic State militants and Kurdish forces around the town of Ayn al-Arab, known to the Kurds as Kobane, the BBC reports.
Syrian activists say at least 300 Kurdish militiamen have crossed in the opposite direction to fight against Islamic State.
IS controls large areas of Syria and Iraq and has seized dozens of villages around Kobane.
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 yesterday.
He was speaking from the southern Turkish province of Sanliurfa, where many of the refugees have sought shelter.
Head of Syria's Kurdish Democratic Union, Mohammed Saleh Muslim, has appealed for international assistance in the battle against the jihadists.
"Kobane is facing the fiercest and most barbaric attack in its history," Reuters news agency quoted him as saying.
"Kobane calls on all those who defend humane and democratic values... to stand by Kobane and support it immediately. The coming hours are decisive," he added.
BBC correspondents say the capture of the town would give IS control of a large strip of Syria's northern border with Turkey.