Turkey has begun to close some of its border crossings with Syria after about 130,000 Kurdish refugees entered the country over the weekend.
On Sunday Turkish security forces clashed with Kurds protesting in solidarity with the refugees. Some protesters were reportedly trying to go to Syria to fight Islamic State (IS).
Most refugees are from Kobane, a town threatened by the advancing militants.
IS has taken over large swathes of Iraq and Syria in recent months.
Before the latest influx, there were already more than one million Syrian refugees in Turkey. They have fled since the start of the uprising against Syrian President Bashar al-Assad three years ago.
Some of the new arrivals are being sheltered in overcrowded schools, as Turkey struggles to cope with the influx.
On Friday Turkey opened a 30km (19-mile) section of the border to Syrians fleeing the town of Kobane, also known as Ayn al-Arab.
But on Monday only two out of nine border posts in the area remained open, the UN refugee agency UNHCR said.
Mark Lowen, BBC News, southern Turkey
Turkey was already struggling to cope with over a million Syrian refugees - that was before 130,000 more fled across the border at the weekend. The sheer numbers would overwhelm any country.
Added to that, they are Kurds from Syria, many of them deeply hostile to Turkey. For 30 years, Turkish forces fought Kurdish rebels in a civil war that killed 40,000 people.
The fact that Turkey is accepting tens of thousands of Kurds is a sign of how allegiances are being forced to change with the onslaught of Islamic State. But deep-seated tension between Kurds and Turks have again come to the surface, leading to Sunday's border clashes.
Turkey fears that Kurds will cross into Syria to join the Kurdish militia. The worry is that, renewed by fresh recruits, it could ally with the outlawed PKK and launch attacks on Turkish soil. All the regional complexities, added to the refugee influx, make for a precarious situation here.
Clashes broke out on Sunday after a demonstration by Kurds on the Turkish side of the border.
Some protesters threw stones at security forces, who responded with tear gas and water cannon. There were no reports of serious injuries.
Turkish security forces were trying to stop Kurdish fighters from entering Syria to take part in the defence of Kobane, says the BBC's Mark Lowen at the scene.
The Kurdistan Workers' Party (PKK), a banned militant group that fought a civil war for autonomy within Turkey for decades, has called on Kurds to join the fight against IS.
Our correspondent says the Syrian conflict has reawakened old hostilities and shaken a fragile peace between Kurds and Turkish authorities.
PKK-affiliated forces have been battling IS in northern Iraq for months.
Carol Batchelor, a spokeswoman for the UN refugee agency in Turkey, told the BBC that the country needed urgent aid to cope with the influx - including food, blankets and winter clothing, "particularly for children".
In other developments:
- A Syrian government air strike killed at least 42 civilians in rebel-held territory in the north-western province of Idlib on Sunday, activists said
- IS issued a statement urging its supporters to kill "disbelievers" from the countries that have joined a international coalition against the group
- US Secretary of State John Kerry and Iranian counterpart Mohammad Javad Zarif discussed the threat posed by IS during a meeting in New York on Sunday, a US official said
IS is closing in on the predominantly Kurdish town of Kobane, having seized dozens of villages in the area in recent days.
It began the assault on Tuesday, and by Sunday militants were about 10km (six miles) away, said the Syrian Observatory for Human Rights.
Reports suggest that IS has used heavy weaponry, including tanks, in the attack.
The US has said it will attack the group in Syria as part of a strategy to destroy it, though so far it has carried out air strikes against IS only in Iraq.
Attacking IS in Syria is considered more complicated, partly because of the strength of the country's air defence system and because foreign strikes do not have the approval of President Assad.
Former UK Prime Minister Tony Blair told the BBC on Sunday that air strikes alone might not be enough to contain the group.
"If necessary, we shouldn't rule out the use of, particularly, special force capabilities," he said.
President Obama has previously ruled out the involvement of US ground troops, and has instead promised to provide arms and training to local forces fighting against IS.
- 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