The plea to Ankara by Staffan de Mistura, a Swedish-Italian diplomat appointed as UN envoy on Syria in July, was a highly unusual one. Normally, the United Nations strives to be neutral in conflicts.
But de Mistura said he was speaking out because of the imminent danger confronting Kobane.
He drew parallels to the 1995 Bosnian war massacre of 300 men and boys in Srebrenica when UN peacekeepers failed to intervene, insisting the world never again could allow something like that to happen.
Reports say the IS jihadists have overrun the headquarters of the Kurdish forces in Kobane after a three-week offensive.
Showing a satellite image to reporters in Geneva, de Mistura said Kobane, which sits just a few kilometres (miles) from the Turkish border, was "literally surrounded" except for one narrow entry and exit point.
There were up to 700 mainly elderly civilians still inside the city centre and another 10,000-13,000 gathered nearby, all of whom were at risk, he said.
"If this falls the 700, plus perhaps if they move a little bit further, the 12,000 people ... will be most likely massacred," he warned.
"We would like to appeal to the Turkish authorities in order to allow the flow of volunteers at least, and their equipment to be able to enter the city to contribute to a self-defence operation," de Mistura said.
He said the volunteers should be allowed to go "with sufficient equipment to be able to fight and defend Kobane", but refused to say whether he thought Turkey should provide weapons.
TURKEY PUSHING FOR BUFFER ZONE
US-led aircraft pounded Islamic State militants near Kobane on Thursday, but as fighting killed dozens, calls grew for ground action to support Kobane's beleaguered Kurdish defenders.
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Turkey especially has come under increasing pressure from the West to help in the fight against jihadists seeking to capture the border town. But Ankara has set down preconditions, including the creation of a buffer zone to protect its security and house some of the more than 1.5 million Syrian refugees already in the country.
De Mistura called on Turkey, "if they can, to support the deterrent actions of the coalition through whatever means from their own territory".
He said he understood Ankara's preconditions, but stressed that reaching international agreement on them "requires time, and Kobane, in our own opinion does not have enough time".
Since the IS assault on Kobane began in mid-September, nearly 500 people have been killed in and around the town and 300,000 have fled the region.
WINDOW OF OPPORTUNITY
Kobane, also known as Ain al-Arab, would be a major prize for the Islamic State group, giving it unbroken control of a 400-kilometre stretch of the border, de Mistura said.
"What would be next? Other villages, Aleppo?" he asked.
He warned that if the jihadists were allowed to continue their murderous rampage "all of us, including Turkey, will be regretting deeply that we have missed an opportunity of stopping" them.
The militants who have torn across large parts of Iraq and Syria have sent shockwaves through the international community as they have committed highly publicised brutal murders, including beheadings of several Western hostages.
De Mistura, who is the third UN envoy appointed to try to help pave the way for a political solution to Syria's three-and-a-half-year bloody civil war, said the horrific rise of IS could actually help Damascus and other opposition groups to find some common ground, for instance on local ceasefires.
He stressed that there were no immediate plans for a new international peace conference for Syria, where more than 191,000 people have died and millions have fled their homes, but said opposition to the IS group was creating "a window we should not be missing".
De Mistura said he and the UN were not yet in talks with the IS group, but stressed that in his line of work he had to be prepared "to talk to anyone".