Story highlights US officials called the attack "unusual"

State Department said it is being investigated

(CNN) The main US ally fighting ISIS in Syria was bombed by Syrian warplanes for the first time Thursday while US military advisers were "nearby," a US defense official told CNN.

The attack against the Kurdish YPG, which took place in Hasakah province in northern Syria, was announced by the YPG and confirmed to CNN by US defense officials. The US has approximately 300 special operations forces in Syria.

The YPG said that in addition to the airstrikes "regime forces randomly targeted the city with artillery and heavy gunfire -- killing and injuring tens of civilians," according to a statement the group posted on Facebook.

The YPG has been critical to US efforts to combat ISIS in Syria, helping to drive the terrorist group out of Manbij in recent days.

During the five-year civil war, the Syrian government has largely refrained from striking the Kurdish group, which has established an autonomous area of control in the country's north. The YPG has primarily battled ISIS as part of the Syrian Democratic Forces, which the US has supplied the with arms and training. US special operations forces were photographed in May alongside YPG troops in Syria.

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