Tens of thousands of Kurdish refugees have fled to Syria-Turkey border region of Kobani to escape onslaught of Islamist militants

Kurdish fighters from Turkey and Iraq are scrambling to help defend a vital Kurdish safe haven in northern Syria, where tens of thousands of Kurds have fled after an offensive by Islamic State (Isis) militants.

The border region of Kobani, home to half a million people, has held out for months against an onslaught by Islamists seeking to consolidate their hold over swaths of northern Syria. But in recent days, Isis extremists have seized a series of settlements close to the town of Kobani itself, sending as many as 100,000 mostly Kurdish refugees streaming across the border into Turkey.

"I don't think in the last three and a half years we have seen 100,000 cross in two days," the representative for the UN refugee agency (UNHCR) in Turkey, Carol Batchelor, told Reuters. "So this is a bit of a measure of how this situation is unfolding, and the very deep fear people have about the circumstances inside Syria and, for that matter, Iraq."

A Kurdish commander on the ground said Isis had advanced to within 9 miles (15km) of Kobani.

A Kurdish politician from Turkey who visited Kobani on Saturday said locals told him Isis fighters were beheading people as they went from village to village.

"Rather than a war this is a genocide operation … They are going into the villages and cutting the heads of one or two people and showing them to the villagers," Ibrahim Binici, a deputy for Turkey's pro-Kurdish People's Democratic party (HDP), told Reuters.

"It is truly a shameful situation for humanity," he said, calling for international intervention. Five of his fellow MPs planned a hunger strike outside UN offices in Geneva to press for action, he said.

The Kurdistan Workers party (PKK), a rebel group that has spent three decades fighting for autonomy for Turkey's Kurds, renewed a call for the youth of Turkey's mostly Kurdish south-east to rise up and rush to save Kobani.

"Supporting this heroic resistance is not just a debt of honour of the Kurds but all Middle East people. Just giving support is not enough, the criterion must be taking part in the resistance," it said in a statement on its website. "[Isis] fascism must drown in the blood it spills … The youth of North Kurdistan [south-east Turkey] must flow in waves to Kobani," it said.

Hundreds of people gathered in solidarity for a third day on the Turkish side of the barbed wire border fence near the town of Suruc, where many of the refugees have crossed. Security forces trying to maintain order fired teargas and water cannon and some protesters started throwing stones at them in frustration.

Even by the standards of Syria's bitter war, the refugee numbers are alarming. Their numbers add to the 2.8 million Syrians who have become refugees in the past three years, and another 6.4 million who have been displaced within their own country – approaching half of Syria's pre-war population of 23 million.

UNHCR and the Turkish authorities said they were preparing for the possibility of hundreds of thousands more refugees arriving in the coming days.

Kobani's relative stability through much of Syria's conflict meant 200,000 internally displaced people were sheltering there before Isis's advance, UNHCR said.

"This massive influx shows how important it is to offer and preserve asylum space for Syrians as well as the need to mobilise international support to the neighbouring countries," said Antonio Guterres, the UN high commissioner for refugees.