Turkey has resisted appeals to use troops to fight the militants, and Syrian Kurds have said they don't want direct Turkish military intervention. Arab members of the US-led coalition against the Islamic State are also opposed to military intervention by Turkey.
Turkish officials agreed last week to support efforts to train and equip the US-backed opposition fighting the militants and Syrian President Bashar Assad is considering a request to allow coalition aircraft to use its Incirlik air base.
The UN appeal came as pitched battles continued in Kobane, where Kurdish fighters have been battling the Islamic State for three weeks.
US airstrikes, including 16 on Thursday and Friday, have helped defenders hold their ground.
Anwar Muslim, the Prime Minister of the canton, said the intensification of strikes in recent days had "been very helpful and we thank the coalition".
The fate of Kobane is a growing dilemma for the Obama administration, which has said its airstrike strategy is designed to obliterate the Islamic State's infrastructure and command system in Syria, rather than to intervene in local battles.
But near-constant media coverage of the town's fight for survival has transfixed international attention and increased pressure on Washington to do more.
De Mistura indicated the end was imminent if help did not arrive quickly.
If Kobane fell, civilians in and around the town "will most likely be massacred", de Mistura said.
With a prewar population of about 45,000, Kobane is the self-proclaimed capital of a wider Kurdish district of about 400,000 people, de Mistura said.
Nearly half of those had fled into Turkey in recent weeks, before the Turkish military sealed the border with tanks and troops.
De Mistura said about 700 civilians were still in the town, and about 12,000 non-combatants were sitting at the border but blocked by Turkey from crossing.
US State Department spokesman Marie Harf said on Friday that although "the number remains low ... it's difficult to ascertain".
Meanwhile, the US is warning of possible attacks against Western targets in reprisal for its military campaign against the Islamic State in Syria and Iraq.
"Authorities believe there is an increased likelihood of reprisal attacks against US, Western and coalition partner interests throughout the world," the notice said.
Isis has "called on supporters to attack foreigners wherever they are".
The US State Department also warned its citizens that "kidnappings and hostage events involving US citizens have become increasingly prevalent", after the kidnapping and murder by Isis of two US journalists and two British aid workers.
"Credible information indicates terrorist groups also seek to continue attacks against US interests in the Middle East and North Africa," the department said.
Assad's forces continue to fight opposition forces, but the Syrian civil war - now in its fourth year - has been eclipsed by the Islamic militants' battle to overrun the city of Kobane.
In recent months, the Islamic extremists have taken control of territory across Iraq and Syria at lightning speed.
- AP
