Turkey shelled Kurdish targets in Syria Sunday despite calls by France and the U.S. to halt such actions. The Turkish military opened a new front against Kurdish forces Saturday after the Kurdish People’s Protection Units (YPG) captured positions in the northern Syrian city of Aleppo.

While the U.S. considers the YPG an ally in the war on the Islamic State group, aka either ISIL or ISIS, Turkey considers it to be closely allied with the outlawed Kurdistan Workers’ Party (PKK). The PKK has carried out an insurgency against the Turkish government for decades.

U.S. Vice President Joe Biden spoke with Turkish Prime Minister Ahmet Davutoğlu by phone Sunday, calling for an end to the artillery strikes on the Kurdish militants. Echoing the appeal made by the U.S., France also demanded “an immediate halt to the bombing, both that of the regime and its allies throughout the country and that of Turkey in the Kurdish zones,” Agence France-Presse reported.

The Syria-based YPG is the armed wing of the Kurdish Democratic Union Party (PYD). The Syrian Civil War has emboldened Kurdish fighters, who now control considerable swaths of war-ravaged Syria. Turkish officials fear the empowered Kurdish forces within Syria could bolster Kurdish forces within Turkey, where they have issued fresh calls for self-rule in recent months.

The YPG managed to seize the Menage air base and several other key positions in Syria from Islamist rebels in recent days. Turkey has demanded it withdraw.

A tenuous ceasefire between the Turkish government and Kurdish militants in the country unraveled last summer amid spillover from the war in Syria. Since then, Turkey has launched an aggressive airstrike campaign against PKK targets in both Iraq and Turkey’s restive southeastern region, which is predominantly Kurdish. The recent developments in Syria has stirred concern that fighting between the Turkish forces and Kurdish militants could spread.

Human rights groups have raised alarms over the massive humanitarian crisis created by Turkey’s military campaign targeting Kurdish fighters, as civilian casualties have climbed in recent months.