Turkey said Monday that some 130,000 people had flooded across its border from Syria fleeing an advance by Islamic State group jihadists on a strategic Kurdish border town.
The massive influx comes as jihadists advance towards Ain al-Arab, known as Kobane to the Kurds, Syria's third-largest Kurdish town.
They have seized dozens of villages in their advance and there have been reports of executions in areas now under their control.
Syrian opposition officials and Kurdish activists have called for international intervention by the US-led coalition assembled to fight IS, but there has been no sign yet of Washington expanding its air campaign in Iraq to Syria.
The IS group has seized large parts of Syria and Iraq, declared an Islamic "caliphate" in areas under its control and committed widespread atrocities including beheadings and crucifixions.
They have murdered two US journalists and a British aid worker in on-camera beheadings, and on Monday an IS spokesman called for the killing of Western citizens whose countries have joined a US-led coalition against the jihadists.
The IS advance towards Ain al-Arab began on Tuesday night, with the militants swiftly gaining ground, prompting a mass exodus to the Turkish border.
Ankara has opened its door to the fleeing Syrian Kurds, with Turkey's Deputy Prime Minister Numan Kurtulmus saying Monday that the number of arrivals was now more than 130,000.
The refugees crossing the border, the vast majority of them from Syria's Kurdish minority, described their terror as IS militants seized their villages.
"They said in the mosques that they could kill all Kurds between seven and 77 years old," Sahab Basravi told AFP.
"So we collected our things and left, immediately."
The Syrian Observatory for Human Rights, a Britain-based monitoring group, said Monday that IS had seized at least 64 villages and executed at least 16 Kurds.
It said the fate of at least 800 residents of villages captured by the group was also unknown.
IS hopes to seize Ain al-Arab to secure its grip over a long stretch of Syria's northern border with Turkey.
It has battled Kurdish forces in several parts of northern and western Syria, viewing the minority as apostates, even though they are also Sunni Muslims, because of their secular outlook.
Across the border in Turkey, the PKK Kurdish rebel group called on Kurds to cross into Syria to help battle IS.
It urged "mobilisation," a pro-Kurdish news agency reported, saying "the day of glory and honour has arrived."
"We call on our entire people, as well as our friends, to step up the resistance," the PKK statement said.
Turkey, the United States and the European Union consider the PKK a "terrorist" group, though Ankara entered peace talks with the organisation two years ago that have now stalled.
The group has joined forces with Kurdish units fighting IS in both Iraq and now Syria, and the main Syrian Kurdish militia, the People's Protection Units (YPG), is often described as close to the PKK.
On Sunday, security forces from Turkey clashed with Turkish Kurds holding a solidarity demonstration on the border, using tear gas and water cannons.
Several hundred Kurdish fighters have crossed into Syria to join the fight, but activists say they stand little chance against IS's heavy weaponry.
Syria's opposition National Coalition and Kurdish officials and activists have urged Washington and other members of an anti-IS coalition to intervene, including with air strikes.
But while Washington has said it would consider strikes against IS in Syria, even without permission from Damascus, its UN envoy said Sunday "no decisions" had been taken.
Ambassador Samantha Power also predicted that Washington "will not do the air strikes alone if the president decides to do the air strikes".
Some 40 countries have signed up to the US-led coalition against IS, and on Sunday US Secretary of State John Kerry discussed the threat posed by the group in rare high-level talks with his Iranian counterpart in New York.
In a statement posted online on Monday, IS spokesman Abu Mohamed al-Adnani said Muslims should seek out and kill Westerners whose countries have joined the coalition, in particular Americans and the French after their countries carried out strikes in Iraq.
"If you can kill a disbelieving American or European... including the citizens of the countries that entered into a coalition against the Islamic State, then rely upon Allah, and kill him," he said in a statement posted online.
"Smash his head with a rock, or slaughter him with a knife, or run him over with your car, or throw him down from a high place, or choke him, or poison him," the IS spokesman said.