Elements belonging to the Kurdish People’s Protection Units (YPG) say Turkish forces have shelled their positions with chemical weapons north of the Syrian city of Manbij.

The Rojava Defense Units released photos of the aftermath of the alleged attacks Thursday, though claims that chemical weapons were used have yet to be corroborated. Turkish state media agency Anadolu confirmed Turkish forces fired upon the YPG after allegedly obtaining intelligence that Kurdish forces were advancing on the nearby city of Jarablus. Turkey and its Free Syrian Army (FSA) allies, backed by U.S. air support, took Jarablus from Islamic State forces Wednesday in Operation Euphrates Shield.

#BREAKING Rockets fired frm #Jarablus hit civilians in rural #Manbij, causing irritation/itching after gas emissions pic.twitter.com/CrJDByGw9V — Rojava Defense Units (@DefenseUnits) August 25, 2016

Turkish Foreign Minister Mevlut Cavusoglu warned that if YPG forces did not move back east of the nearby Euphrates river, they would take “necessary measures.”

U.S. Secretary of State John Kerry reassured Turkey Thursday that YPG forces would withdraw so Turkish forces could take Jarablus, but reports show at least some of the Kurdish forces failed to do so. (RELATED: These Two US Allies Have Turned On Each Other Instead Of Fighting ISIS)

Turkey has a long and bloody history with its own Kurdish minority, and the U.S. ally considers the YPG a terrorist force. Turkish leaders have long been concerned over the rapid advance of Kurdish forces fighting ISIS along its southern border, allowing the YPG to take Jarablus would be unacceptable from Turkey’s perspective.

Turkey’s fight with the YPG puts the U.S. in a difficult diplomatic position, as both actors are essential partners in the fight against ISIS. YPG forces, which make up a crucial portion of the Syrian Democratic Forces (SDF), have been regularly supported by the U.S. in the past. Turkey controls the Incirlik air base, which the U.S. uses frequently in anti-ISIS operations.

Pentagon Press Secretary Peter Cook urged both sides to focus the fight on the fight against ISIS during a press briefing Thursday.

“ISIL (ISIS) poses a threat to all of us,” Cook said, adding that sharing ISIS as a common enemy is a major reason why all parties were initially supportive of the Jarablus operation.

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