Turkey has begun to close some of its border crossings with Syria after about 100,000 Kurdish refugees entered the country over the past two days.
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, Turkey had taken in more than 847,000 refugeessince the uprising against Syrian President Bashar al-Assad began 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.
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.
Islamic State (IS) is closing in on the predominantly Kurdish town, 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 air 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.