The massive influx came as 49 Turks kidnapped by IS jihadists in Iraq in June were freed and returned to Turkey for emotional reunions with their families.

On Saturday, Turkey's deputy prime minister Numan Kurtulmus said at least 45,000 Syrian Kurds had crossed into the country.

"As of now, 45,000 Syrian Kurds have crossed the border and entered the Turkish soil from eight entrance points," he said.

The exodus was prompted by intense clashes between the Islamic State (IS) group and Kurdish fighters trying to hold off an assault on the town of Ain al-Arab, known as Kobane by the Kurds.

It is the third largest Kurdish town in Syria and a strategic prize because it lies on the border with Turkey in northern Aleppo province.

Since Tuesday night, IS fighters have been advancing around the town, hoping to seize it and secure their control over a large swathe of Syria's northern border with Turkey.

The group has moved quickly, seizing at least 60 villages around the town, although the Syrian Observatory for Human Rights, a Britain-based monitor, said 18 IS fighters were killed in clashes overnight.

On Saturday, the Observatory said 300 Kurdish fighters had entered Syria from Turkey to reinforce the Syrian Kurdish People's Protection Units (YPG) fighting IS.

- Children, elderly flee fighting -

But the clashes have forced tens of thousands of civilians to flee, with most seeking shelter in Turkey.

Those arriving on Saturday were all ages -- a child in a pink top clutching a sibling's hand as they marched across the dusty border, an elderly woman in a wheelchair being helped by relatives.

Turkish forces cut the barbed wire curled along the frontier in a bid to make it easier for the incoming civilians to cross.

IS's advances have prompted calls from Syria's opposition and Kurdish officials for international intervention, with one leader warning of "ethnic cleansing".

Overnight, the opposition National Coalition urged international air strikes to "stop mass atrocities" if IS advances into Ain al-Arab.

"Air strikes are needed to help opposition forces protect vulnerable civilians," the coalition's US representative Najib Ghadbian said.

And Salih Muslim Mohamed, a leader of the Democratic Union Party (PYD), urged the United States and Europe to help Ain al-Arab/Kobane avoid the fate of the Iraqi town of Sinjar, which has been emptied of its Yazidi minority residents in the wake of an IS onslaught.

"Kobane is facing the most barbaric attack in its history," he warned.

"If you want to avoid an ethnic cleansing even more barbaric than that in Sinjar, you must support Kobane because the next few hours will be decisive," he added in a statement late Friday.

"Kobane is waiting for your urgent action."

The Observatory reported on Saturday that IS militants had executed at least 11 Kurds, and that the fate of some 800 residents who fled their villages remained "unknown".

- Turkish hostages return home -

As Turkey dealt with the arrivals on its southern border, the government in Ankara welcomed home 49 Turkish citizens kidnapped by IS militants in the Iraqi city of Mosul in June.

It was unclear how the group, kidnapped from Turkey's consulate in Mosul, had been freed, though President Recep Tayyip Erdogan referred to a "secret operation".

There were emotional and triumphant scenes on their return to Turkey, with Prime Minister Ahmet Davutoglu cutting short a trip to Azerbaijan to greet the former hostages.

"There are unnamed heroes, like those who brought our citizens back to Turkey. They acted for the sake of our country, for the sake of our people. I salute them," he told a cheering crowd of supporters at Ankara's airport.

The United States has organised a coalition of countries to tackle IS jihadists who have declared an Islamic "caliphate" in parts of Syria and Iraq and carried out abuses including beheadings and crucifixions.

US President Barack Obama plans to make his case against IS before the world at the UN General Assembly next week in a bid for more international support.

"We won't hesitate to take action against these terrorists in Iraq or in Syria," Obama said of the militants. "But this is not America's fight alone."

burs-sah/srm