BEIRUT (Reuters) - At least 90 Turkish rockets pounded a group of Kurdish fighters allied to a U.S.-backed militia in northern Syria on Friday, as clashes intensified between two sides both supposed to be fighting Islamic State, a monitor and militia spokesperson said.

The confrontation between Turkey-backed Syrian rebels and Kurdish fighters allied to a U.S.-backed militia fighting Islamic State is escalating as both sides race to be the first to expel Islamic State from the northern Syrian city of al-Bab.

On Wednesday the heaviest Turkish air strikes so far on the Kurdish fighters hit villages recently captured from Islamic State by the Kurdish fighters, highlighting the conflicting agendas of NATO members Ankara and Washington in an increasingly complex battlefield

The United States has backed the Kurdish-led Syrian Democratic Forces (SDF) in their fight against Islamic State, infuriating Ankara, which sees the umbrella group’s dominant YPG militia as an extension of Kurdish PKK militants who have waged a three-decade insurgency in southeastern Turkey.

Turkey, a major backer of the insurgency against President Bashar al-Assad, entered the Syrian conflict in August, using its armour and air power to help Free Syrian Army rebel groups take territory near the border held by Islamic State.

But it fears the YPG will try to connect three de facto autonomous Kurdish cantons that have emerged during the five-year war to create a Kurdish-run enclave in northern Syria, stoking the separatist ambitions of Kurds on its own soil.

Its intervention therefore also aimed to prevent Kurdish forces from gaining more ground.

An adviser to the Turkey-backed FSA alliance who gave his name as Osama Abu Zayd told Reuters Friday’s clashes were fierce and were widening as they try to push the Kurdish fighters out of the northern Aleppo countryside.

“Two days ago the (Kurdish fighters) tried to exploit our battle against Daesh (Islamic State) to advance towards Marea”, he said. Marea is a town in Turkey-backed rebel territory on the way to al-Bab.

“What is happening today is a natural response to these separatist groups,” Abu Zayd said.

Ahmad Araj, a political representative for the Kurdish fighters allied to the U.S.-backed SDF militia told Reuters their fighters were now under attack both from Islamic State and from Turkey.

He said more than 150 rockets on Friday hit areas they had pushed Islamic State out of this week.

“Today at 10 am Turkish shelling began ... there was an attack and the clashes have continued since morning,” he said. “Their rockets are not targeting Islamic State, rather they are targeting our forces in areas liberated (from Islamic State).”

The Syrian Observatory for Human Rights said more than 90 rockets fired by Turkish forces hit the town of Sheikh Issa and other areas along the frontline between territory held by the two sides in northern Aleppo province on Friday.

Turkish Prime Minister Binali Yildirim said on Friday Turkey’s activities in Syria are aimed at destroying terrorist organisations and securing its border, and it discusses all of its operations with coalition partners.