At least 500 civilians trapped in the besieged border town of Kobane face being "massacred" should it fall to ISIL, the United Nations envoy to Syria warned Friday as the U.S.-led coalition launched its most intense day of strikes to date.

The trapped civilians in the Kurdish town "will be most likely massacred" if the city falls to the extremists, said U.N. envoy Staffan de Mistura.

He said a U.N. analysis of the battle in Kobane shows that only a small portion of the town remains open for people to enter or flee. He said there were about 500 to 700 civilians still trapped there while 10,000 to 13,000 remain stuck in an area nearby, close to the border.

Islamic State in Iraq and the Levant (ISIL) fighters on Thursday managed to capture a police station to the east of Kobane, a city at the center of battles between the group and the U.S.-led coalition. The United Kingdom-based Syrian Observatory for Human Rights, which monitors the Syrian civil war, said the station was later hit by coalition jets and destroyed.

De Mistura, who spoke to reporters in Geneva, invoked the genocides in Rwanda in 1994 and in the Bosnian town of Srebrenica in July 1995 as he appealed to the world to prevent another catastrophe.

He called on Turkish authorities to allow volunteers and equipment to flow into the town and help its Syrian Kurdish defenders. Without more such help, he added, Kobane is "likely to fall."

The onslaught has already forced more than 200,000 to flee across the border into Turkey. Rami Abdurrahman, the Observatory's chief, said more than 500 people have been killed in and around Kobane.

Overnight airstrikes destroyed an ISIL training camp and two combat unites. More than 20 airstrikes have been conducted in the area since Wednesday afternoon, according to the Observatory.

On Friday, ISIL fighters struck back and shelled a Syrian border crossing with Turkey in efforts to capture it and cut off Kobane. The Observatory said ISIL is now in control of more than third of the strategic Kurdish town that is only miles away from Turkish soil.

Idriss Nassan, an official with Kobane's Kurdish government, contested that report.

"I can confirm that they don't control a third of the city. There is only a small part of Kobane under the control of Daesh," said Nassan, using the Arabic name to refer to ISIL.

He said Kurdish fighters managed to regain several other town areas on Thursday, adding that ISIL also brought reinforcements from their stronghold in the border town of Jarablous and Manbij.