A UN envoy has called on Turkey to help prevent a slaughter in the Syrian border town of Kobani at the hands of Islamic State (IS) fighters, saying he feared a repeat of the 1995 Srebrenica massacre when thousands died.

Staffan de Mistura, the UN's envoy to Syria, appealed to Ankara to let volunteers cross the frontier so they can reinforce Kurdish militias defending the town that lies within sight of Turkish territory.

He revived memories of the break-up of Yugoslavia when Bosnian Serb forces marched into the town of Srebrenica, which was supposed to be under UN protection, and gunned down more than 8,000 Muslim men and boys at execution sites.

Mr de Mistura said: "Do you remember Srebrenica? We do. We never forgot and probably we never forgave ourselves."

Turkey has stationed tanks on hills overlooking Kobani but has so far refused to intervene without a comprehensive deal with the US and other allies on the Syrian civil war. It has also prevented Turkish Kurds from crossing the frontier to reinforce their fellow Kurds defending the town.

Mr de Mistura said: "We would like to appeal to the Turkish authorities to allow the flow of volunteers at least and their own equipment in order to be able to enter the city and contribute to a self-defence action."

Saying Kobani is likely to fall "if left unattended", he said self-defence with sufficient equipment to do so was an international human right.

While much of the town's population has already fled the IS offensive, between 500 and 700 people, most of them elderly, are still sheltering in Kobani.

Asked if he meant Turkey should allow arms supplies, Mr de Mistura replied: "I said what I said: equipment can be many things."

In addition to the people in Kobani, between 10,000 and 13,000 are nearby in a border area between Syria and Turkey, and there is one last exit point out of the town.

The UN envoy warned: "If this falls, the 700, plus perhaps the 12,000 people, apart from the fighters, will be most likely massacred.

"When there is an imminent threat to civilians, we cannot, we should not, be silent."

Former Bosnian Serb leader Radovan Karadzic is on trial in the Hague for war crimes, crimes against humanity and genocide, including the massacre at Srebrenica, Europe's worst such incident since the Second World War.

Mr De Mistura, who followed Kofi Annan and Lakhdar Brahimi into the role of UN Syria mediator, ­reiterated there was no military solution to the Syrian war.

And he said facts on the ground had changed since an international Geneva conference agreed ground rules for ending the conflict in 2012.

He said nobody would win the war and the emergence of IS as a common enemy had given a chance for local ceasefires and ending long-running sieges in Syria.

Asked if he would negotiate with IS, he said he was "authorised and expected to talk to anyone" if that produced a political solution or a humanitarian relief.

He added: "But I am not proposing, I am not planning, and they are not asking to meet any one of us."

IS fighters advanced deeper into Kobani yesterday and now control 40 per cent of the town including the security quarter used by the local government.