Thousands of people most likely will be massacred if Kobani falls to Islamic State group fighters, a U.N. envoy said Friday, as militants fought deeper into the besieged Syrian-Kurdish town in full view of Turkish tanks that have done nothing to intervene.

U.N. envoy Staffan de Mistura said Kobani could suffer the same fate as the Bosnian town of Srebrenica, where 8,000 Muslims were murdered by Serbs in 1995, Europe's worst atrocity since World War II, while U.N. peacekeepers failed to protect them.

"If this falls, the 700, plus perhaps the 12,000 people, apart from the fighters, will be most likely massacred," de Mistura said. The United Nations believes 700 mainly elderly civilians are trapped in the town itself and 12,000 have left the center but not made it across the border into Turkey.