This follows clashes on Sunday between Turkish security forces and protesting Kurds. The closures may stop Kurdish fighters entering Syria to fight IS.Most refugees are from the town of Kobane, where a massacre by the approaching militants is feared.IS has taken over large swathes of Iraq and Syria in recent months.Turkey has taken in more than 847,000 refugees since 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.'Huge numbers'Turkey opened its border on Friday to Syrians fleeing the town of Kobane, also known as Ayn al-Arab.Clashes broke out on Sunday after a solidarity demonstration by Kurds on the Turkish side of the border.Some protesters are believed to have thrown stones at security forces, but there were no immediate reports of injuries.Reports say Turkish security forces were trying to stop Kurdish fighters from entering Syria to take part in the defence of Kobane.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 around 10 km (six miles) away, said the Syrian Observatory for Human Rights.Reports suggest that IS has used heavy weaponry, including tanks, in the attack.The UN refugee agency UNHCR says it is boosting relief efforts as hundreds of thousands more refugees could try to cross the border."Turkey is assisting with all needs but it's huge numbers," said UNHCR spokeswoman Selin Unal.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.(BBC)Bakudaily.Az