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CNN team on the Turkey-Syria border describe fierce fighting for the city of Kobani

A Kurdish fighter says it will be "impossible" to hold Kobani if current conditions continue

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(CNN) The battle for a key Syrian city on the Turkish border intensified Monday as ISIS continued its push for control of Kobani while also scoring a victory in neighboring Iraq, taking a strategically important military base in Anbar province.

A fighter from the Kurdish People's Protection Unit, or YPG, told CNN's Arwa Damon that the battle in Kobani concerned the main border crossing into Turkey. If ISIS took control, he said, "it's over."

The fighter said the Kurdish fighters had pushed back an attempted advance by ISIS on Monday morning but that it would be "impossible" for them to hold their ground if current conditions continued.

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Should they take Kobani, the militants would control three official border crossings between Turkey and Syria and a stretch of the border about 60 miles (97 kilometers) long.

Monday has been one of the most violent days in Kobani since ISIS launched its assault on the Syrian city, with sounds of fierce fighting, including gunfire and explosions, CNN staff on the Syria-Turkey border said.

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