The Islamist militant group ISIS launched an attack Saturday on the Syrian border town of Kobani from Turkey, a Kurdish official and activists said, although Turkey denied that the fighters had used its territory for the raid.

The assault began when a suicide bomber driving an armored vehicle detonated his explosives on the border crossing between Kobani and Turkey, said the Britain-based Syrian Observatory for Human Rights and Nawaf Khalil, a spokesman for Syria's powerful Kurdish Democratic Union Party.

ISIS "used to attack the town from three sides," Khalil said. "Today, they are attacking from four sides." Turkey, while previously backing the Syrian rebels fighting to topple President Bashar Assad in that country's civil war, has been hesitant to aid them in Kobani because it fears that could stoke Kurdish ambitions for an independent state.

A Turkish government statement on Saturday confirmed that one of the suicide attacks involved a bomb-loaded vehicle that detonated on the Syrian side of the border. But it denied that the vehicle had crossed into Kobani through Turkey, which would be a first for the extremist fighters. Mustafa Bali, a Kobani-based activist, said by telephone that ISIS fighters have taken positions in the grain silos on the Turkish side of the border and from there are launching attacks toward the border crossing point.

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— The Associated Press