Some 66,000 refugees - mainly Syrian Kurds - have crossed into Turkey in 24 hours, officials say, as Islamic State militants advance in northern Syria.
Turkey opened its border on Friday to Syrians fleeing the Kurdish town of Kobane in fear of an IS attack.
The UN refugee agency said it was boosting relief efforts as hundreds of thousands more could cross the border.
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.
Analysis. By Mark Lowen, BBC News, on the Turkish-Syrian border
The influx is astonishing - and still continues.
At least 66,000 Syrian Kurds have entered Turkey since Friday, when the country opened parts of its border crossing with Syria.
Around 300 Kurdish fighters are said to have gone the other way, crossing from Turkey into Syria to help resist the IS onslaught.
Until recently, Turks and Kurds fought a civil war that killed 40,000 people. The fact that Turkey is now accepting tens of thousands of Kurdish refugees is a sign of how the rise of Islamic State is shifting allegiances in this region.
The UN High Commissioner for Refugees (UNHCR) said in a statement that, along with the Turkish government, it was preparing for the possibility of hundreds of thousands more refugees arriving over the coming days, as the battle for Kobane forced more people to flee.
It added that the town had been living in relative safety for much of the Syrian conflict and as many as 200,000 internally displaced people had found refuge there.
The UK-based Syrian Observatory for Human Rights said at least 300 Kurdish fighters had joined Syrian Kurdish ranks in the Kobane area to fend off the IS advance. The activist group did not specify which Kurdish group the fighters belonged to.
"Islamic State sees Kobane like a lump in the body: they think it is in their way," the observatory's Rami Abdulrahman said.
Syrian activists say IS has seized as many as 60 villages surrounding Kobane since fighting began earlier this week.
The observatory said on Saturday that at least 11 Kurds had been executed by IS, with the fate of some 800 residents who fled the villages "unknown".
The 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.
- Formed out of al-Qaeda in Iraq (AQI) in 2013, IS first captured Raqqa in eastern Syria
- It captured broad swathes of Iraq in June, including Mosul, and declared a "caliphate" in areas it controls in Syria and Iraq
- Pursuing an extreme form of Sunni Islam, IS has persecuted non-Muslims such as Yazidis and Christians, as well as Shia Muslims, whom it regards as heretics
- Known for its brutal tactics, including beheadings of soldiers, Western journalists and aid workers
- The CIA says the group could have as many as 31,000 fighters in Iraq and Syria
- The US has been launching air strikes on IS targets in north-eastern Iraq since mid-August
In a separate development, 46 Turkish and three Iraqi hostages seized by IS have been freed and taken to Turkey after a covert operation led by Turkey's intelligence agency.
The hostages were seized from the Turkish consulate after IS militants overran Mosul in a rapid advance in June.
Few details about the operation have been released, but Prime Minister Ahmet Davutoglu said Turkey's "own methods" brought the group home.
"After intense efforts that lasted days and weeks, in the early hours, our citizens were handed over to us and we brought them back to our country,' Mr Davutoglu said.
The group was greeted by flag-waving crowds in Ankara, after arriving there early on Saturday.
"I can't describe the days we've lived through. I can't describe what we felt, me and my relatives," one of the hostages was quoted as saying after arriving in southern Turkey.
As well as consular employees, children and special forces police were among the hostages.
Thirty countries have pledged to join a US-led coalition against the militants but Turkey has said it will only allow humanitarian and logistical operations from a Nato air base on its soil.
Turkey has come under pressure from Western countries to stem the flow of foreign fighter joining IS.
President Recep Tayyip Erdogan has said Turkey is developing plans for a buffer zone on its border with Iraq and Syria.