BEIRUT — Nearly a week of fighting between Kurds and Arab rebels in northern Syria risks opening a new front in the already bloody battle for control of the country, underscoring the complexity of a conflict that threatens to ignite sectarian and ethnic tensions across the region.

Efforts were underway Wednesday between the leadership of the rebel Free Syrian Army and Kurdish representatives in Turkey to negotiate an end to the clashes, in which scores of hostages have been seized and more than 40 fighters on both sides killed.

But the eruption of violence exposed little-noticed frictions that have been building between the mostly Arab Syrian opposition forces seeking to oust President Bashar al-Assad and Syrian Kurds, who increasingly see the revolt as an opportunity to extend their long-standing aspirations for the autonomy — and perhaps independence — of Kurdish areas across the region.

“This is setting the stage for a conflict that is going to come,” said Joost Hiltermann, deputy director of the Middle East and North Africa program at the International Crisis Group. And unless Syria’s already divided opposition takes steps to address Kurdish aspirations in a meaningful way, chances are high that a wider Arab-Kurdish war could follow any regime change in Damascus, he said.

Two days later, the hostilities spilled into the northern area of Aleppo province from which government forces were driven out by rebels months ago. An exchange of the hundreds of hostages seized by both sides in the Aleppo clashes soured after one of the Kurds was returned dead, bearing marks of torture inflicted by the rebels. Opposition fighters in the border town of Azaz fired shells at the nearby Kurdish village of Qastal Jendo. Early Wednesday, PYD fighters ambushed rebel fighters near the rebel-controlled Turkish border crossing of Bab al-Salameh, killing one and injuring two.

Compounding the complexity of the fight, however, are splits within both sides, which raise questions about whether the violence can be contained.

Col. Malik al-Kurdi, a spokesman for the Free Syrian Army in Turkey, called the attacks on Kurds a “mistake” and said the units involved, which he refused to name, do not fall under the control of the mainstream rebel leadership.

Kurds say that fighters from the extremist group Jabhat al-Nusra, suspected of adherence to the ideologies espoused by al-Qaeda, started the shooting in the city of Aleppo and that another rebel unit that does not fall under the Free Syrian Army’s command, the Northern Storm Brigade, launched the attacks on the Kurdish village from the rebel-controlled town of Azaz.