If ISIS takes control of Kobani, at least 500 civilians trapped in the Syrian town on the Turkish border “will be most likely massacred,” the U.N.’s envoy to Syria warned on Friday. The remarks by Staffan de Mistura in Geneva came as a monitoring group estimated that Islamic State fighters control at least 40 percent of Kobani.

The coalition led by the United States against ISIS has hit the town with airstrikes in recent days. While the military action initially repelled ISIS to the outskirts of Kobani, the fighters have since regrouped. About 500 to 700 civilians, some of them elderly, are trapped in the predominantly Kurdish town on the border with Turkey, Mistura estimated, according to the Associated Press. Another 10,000 to 13,000 civilians are stranded near the Syria-Turkey border, he said.

If Kobani falls to ISIS, those civilians “will be most likely massacred,” Mistura said. The U.N. envoy to Syria also warned that the militant group could further control large swaths of territory on the border if the siege on Kobani is successful.

The town was hit with nine airstrikes by the U.S.-led coalition on Thursday. Six of the strikes hit “a large Islamic State unit and two small ones.” Two buildings held by ISIS were destroyed in the strikes as well as a tank and heavy machine gun, according to U.S. Central Command, which oversees military operations in the area. Another two buildings were destroyed in three airstrikes north of Kobani. Two Islamic State units were also targeted in those attacks.

The monitoring group Syrian Observatory for Human Rights said ISIS controls at least 40 percent of Kobani. Mistura said only few parts of Kobani allow people to enter or flee the besieged town.

The strikes targeting Kobani were conducted on the same day airstrikes were launched in Iraq, where ISIS controls a large amount of territory. A strike south of Sinjar, Iraq, destroyed an ISIS bunker, ammunition cache and a small ISIS unit, Central Command said. Another strike south of Sinjar Mountain destroyed an ISIS armored vehicle and a small ISIS unit.