Militants of the Islamic State group have captured almost 40 percent of the Syrian border town of Kobane, including the headquarters of the Kurdish forces, the Syrian Observatory for Human Rights said, in a statement released Friday.

“The jihadists have taken control of the headquarters buildings,” the Observatory said, adding that airstrikes by the United States-led coalition had failed to prevent the advance of ISIS militants into the Kurdish-populated town.

If confirmed, this would be the most significant advance by the Islamic State group in Kobane, which has been under sustained attack since Sept. 16.

Meanwhile, Staffan de Mistura, the new United Nations envoy to Syria, said that at least 500 civilians trapped in Kobane could be “massacred” if the town is captured by the Islamic State group, according to an Associated Press report.

“Everybody should do whatever they can to stop this,” Mistura reportedly said at a news conference in Geneva on Friday. “I hope we will not see people beheaded.”

Mistura also reportedly urged Turkish authorities to allow Kurdish volunteers to cross the border into Syria to aid and assist those still trapped in Kobane.

The Islamic State group’s onslaught on Kobane has so far forced nearly 200,000 people to flee across the border into Turkey. More than 500 people, including many children, have been killed in the town since the fighting began, according to media reports.