The threat of massacre by the advancing Islamic State looms for civilians trapped in Kobani, the Syrian Kurdish town located near the Turkish border, a United Nations official said Friday.

At a press conference in Geneva, the U.N. Syria Envoy Staffan de Mistura warned reporters that Kobani civilians have only a limited possibility of escape. The announcement came as the Islamic State seized what is called the “Kurdish security-quarter,” which includes the city’s police station, the municipality and other local government buildings. Kurdish militiamen have been struggling to defend this area.

“A terrible slaughter is coming. If they take the city, we should expect to have 5,000 dead within 24 or 36 hours,” said Rooz Bahjat, a Kurdish intelligence officer.

Since the Islamic State began their advancement into Kobani in mid-September, at least 200,000 people have fled across the border into Turkey. Nearly 500 people have died, according to activists.

Experts said that U.S. airstrikes have failed to halt the Islamic State’s movement into the city. But a statement from the U.S. Central Command said that recent strikes in Kobani destroyed two Islamic State training facilities, vehicles and tanks.

In an appeal to the international community to prevent the imminent threat of massacre, de Mistura invoked at the U.N. press conference moments in history where many argue the world stood idly by in the face of mass human suffering, like in Rwanda and Srebrenica.

The civilians of Kobani “will be most likely massacred,” de Mistura said. “When there is an imminent threat to civilians, we cannot, we should not be silent.”