Thousands of Syrian refugees fleeing fighting have massed at the Turkish border. Photo: BBC NEWS

Turkish troops are trying to control the flood of refugees, and stop Kurdish fighters crossing into Syria. Photo: BBC NEWS

The UNHCR said this was the largest influx in such a short period since the start of the Syrian conflict in 2011.

The Kurdish refugees are fleeing an advance by Islamic State (IS) militants, who have seized swathes of Iraq and Syria in recent months.

IS fighters are reported to be closing in on the Syrian town of Kobane.

The capture of Kobane, also known as Ayn al-Arabon, would give the jihadists complete control of the area.

Even before the latest influx, Turkey was struggling to cope with more than a million Syrian refugees who have crossed into its territory since the uprising against President Bashar al-Assad began more than three years ago.

Carol Batchelor, the UNHCR's envoy in Turkey, told the BBC that such high numbers of refugees would place a strain on any host community.

Mark Lowen, BBC News, southern Turkey

The sheer number of refugees 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.

She called for "increased solidarity and international assistance" for those flooding across the border.

"The situation is deepening. It's becoming protracted. People are desperately in need," she added.

Ms Batchelor said food, blankets and winter clothing - particularly for children - were needed.

Turkish Deputy Prime Minister Numan Kurtulmus said his country was preparing for "the worst case scenario" - an influx of hundreds of thousands more refugees.

"I hope that we are not faced with a more populous refugee wave but if we are, we have taken precautions," he said, without giving details.

On Friday, Turkey opened a 30km (19 mile) section of its south-west border to Syrian Kurds fleeing Kobane.

Clashes then broke out on Sunday after protests by Kurds on the Turkish side of the border.

The BBC's Mark Lowen, at the scene, said Turkish forces were trying to stop Kurdish fighters from entering Syria to take part in the defence of Kobane.

By Monday only two out of nine border posts in the area remained open, the UNHCR said.

Our correspondent says the Syrian conflict has reawakened old hostilities and shaken a fragile peace between Kurds and Turkish authorities.

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.

PKK-affiliated forces have been battling IS in northern Iraq for months.

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

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.

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.

US President Barack Obama previously ruled out the involvement of US ground troops, and has instead promised to provide arms and training to local forces fighting against IS.