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Lefteris Pitarakis, Associated Press
A Turkish Kurd, bottom right, standing in Mursitpinar, in the outskirts of Suruc, Turkey, on the Turkey-Syria border, uses binoculars to watch intensified fighting over the border in Kobani, Syria, between Syrian Kurds and the militants of Islamic State group, as Turkish Kurds offer their Muslim Friday prayers in support of Syrian Kurds, Friday, Oct. 10, 2014. Kobani, also known as Ayn Arab, and its surrounding areas, has been under assault by extremists of the Islamic State group since mid-September and is being defended by Kurdish fighters.

MURSITPINAR, Turkey — At least 500 civilians who remain trapped in the Syrian Kurdish border town of Kobani are likely to be "massacred" if it falls to the Islamic State group, the U.N. envoy to Syria warned Friday, calling on the world to help avert a catastrophe as the extremists pushed deeper into the embattled town.

Staffan de Mistura raised the specter of some of the worst genocides of the 20th century during a news conference in Geneva, where he held up a map of the town along the Syria-Turkey border and said a U.N. analysis shows only a small corridor remains open for people to enter or flee Kobani.

The dramatic warning came as the Islamic State group pushed into Kobani from the south and east, taking over most of the so-called "Kurdish security quarter" — an area where Kurdish militiamen who are struggling to defend the town maintain security buildings and where the police station, the municipality and other local government offices are located.

The onslaught by the Islamic State group on Kobani, which began in mid-September, has forced more than 200,000 to flee across the border into Turkey. Activists say the fighting has already killed more than 500 people.

"The city is in danger," said Farhad Shami, a Kurdish activist in Kobani reached by phone from Beirut. He reported heavy fighting on the town's southern and eastern sides and said the Islamic State group was bringing in more reinforcements.

U.S.-led airstrikes against the extremists appear to have failed to blunt their push on Kobani. The Britain-based Syrian Observatory for Human Rights said that with the new advances, the Islamic State group was now in control of 40 percent of the town.

The Observatory, which collects its information from a network of activists on the ground in Syria, said a suicide bomber from the Islamic State group blew his car up near the Grand Mosque just west of the security quarter, but there was no immediate word of casualties.

The U.S. Central Command said in a statement that the U.S.-led coalition conducted nine airstrikes in Syria on Thursday and Friday. It said strikes near Kobani destroyed two Islamic State training facilities, as well as vehicles and tanks. Another strike in the eastern province of Deir el-Zour — controlled by the extremists — destroyed an Islamic State armored vehicle staging facility, it said.

On Friday, the militants shelled Kobani's single border crossing with Turkey in an effort to capture it and seal off the town, a local Kurdish official and Syrian activists said.

The official, Idriss Nassan, said Islamic State fighters aim to seize the crossing in order to close the noose around the town's Kurdish defenders and prevent anyone from entering or leaving Kobani.

By midmorning Friday, occasional gunfire and explosions that appeared to be rocket-propelled grenades and mortar shells could be heard from across the border in Turkey, and plumes of smoke were seen rising in the distance. The Observatory said the militants shelled several areas in Kobani, including the border crossing.

"Daesh is doing all it can to take the border crossing point through the farmlands east of the city," Nassan said, using an Arabic acronym to refer to the Islamic State group. "They think there might be help (for the Kurdish militia) coming through the crossing so they want to control the border."

In Geneva, de Mistura said that a U.N. analysis of the situation on the ground shows that only a small portion of the town remains open for people to enter or flee. He said there were 500 to 700 elderly people and other civilians still trapped there while 10,000 to 13,000 remain stuck in an area nearby, close to the border.

De Mistura 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.

If the town falls to the Islamic State fighters, "we know what they are capable of doing," said the Italian-Swedish diplomat, who was appointed to the U.N. post in July.

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."

"You remember Srebrenica? We do. We never forgot. And probably we never forgave ourselves for that," he said. In both Rwanda and Srebrenica, the U.N. had troops on the ground but they failed to save the lives of the civilians they were mandated to protect.

There are no U.N. troops in Syria. Turkey has deployed troops and tanks across the border, but despite U.S. pressure, Ankara has said it will not join the fight unless the U.S.-led coalition also goes after the government of Syrian President Bashar Assad.

De Mistura appealed to Turkish authorities to allow volunteers and equipment to flow into Kobani and help its Syrian Kurdish defenders.

Without more such help, he added, Kobani is "likely to fall."

The fight over Kobani has eclipsed the larger Syrian civil war, where Assad's forces continue to fight rebels seeking to topple him in many parts of the country.

On Friday, activists said at least nine civilians were killed in a government airstrike that targeted the village of Harra in the southern province of Daraa. More than 20 people were also killed a day earlier in government airstrikes in Damascus suburbs, they said.

The Syrian National Coalition, Syria's Western-backed main opposition group, accused Assad of "openly exploiting" the coalition's war against the Islamic State group to continue killing Syrians.

Heilprin reported from Geneva. Associated Press writers Bassem Mroue and Zeina Karam contributed from Beirut.