The Islamic State group 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.

The Islamic State group "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.

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"Claims that the vehicle reached the border gate by crossing through Turkish soil are a lie," read the statement released from the government press office at the border town of Suruc. "Contrary to certain claims, no Turkish official has made any statement claiming that the bomb-loaded vehicle had crossed in from Turkey."

"The security forces who are on alert in the border region have ...  taken all necessary measures," the statement continued.

Associated Press journalists saw thick black smoke rise over Kobani during the attack. The sound of heavy gunfire echoed through the surrounding hills as armored vehicles took up positions on the border. The Observatory said heavy fighting also took place southwest of the town where the Islamic State group brought in tanks to reinforce their fighters.

Mustafa Bali, a Kobani-based activist, said by telephone that Islamic State group 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. He added that the U.S.-led coalition launched an airstrike Saturday morning on the eastern side of the town.

"It is now clear that Turkey is openly cooperating with Daesh," Bali said, using the Arabic acronym for the Islamic State.

The Islamic State group began its Kobani offensive in mid-September, capturing parts of the town as well as dozens of nearby villages. The town later became the focus of airstrikes by the U.S.-led coalition against the militants.

Kurdish fighters slowly have been advancing in Kobani since late October, when dozens of well-armed Iraqi peshmerga fighters joined fellow Syrian Kurds in the battles. The fighting has killed hundreds of fighters on both sides over the past two months.

The Observatory said Saturday the latest fighting killed at least eight Kurdish fighters and 17 jihadis.

The Islamic State group has declared a self-styled Islamic caliphate in areas under its control in Iraq and Syria, governing it according to its violent interpretation of Shariah law. The group has carried out mass killings targeting government security forces, ethnic minorities and others against it.