Thursday's air strike, conducted by two Syrian SU-24 attack planes, targeted Kurdish forces who were undergoing training from US special operations advisers in the area around the northeastern city of Hasakeh, Pentagon spokesman Captain Jeff Davis said.
In a bid to intercept the Syrian jets, the coalition scrambled its own jets to the area, but the regime planes had left by the time they arrived.
"This was done as a measure to protect coalition forces," Davis said.
But the warning appeared to fall on deaf ears -- Syrian regime warplanes bombarded Hasakeh for a second day on Friday.
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As soon as Thursday's strike began, ground forces tried to hail pilots via radio -- to no avail.
US forces then contacted Russia, which has been bombing parts of Syria for nearly a year in support of President Bashar al-Assad, but Russian military officials said the planes were Syrian.
No coalition injuries were reported in Thursday's strike, and US forces have been moved from the area and are in a safe location, Davis said without elaborating.
The coalition is now conducting additional combat air patrols in the region, he added.
The Kurdish People's Protection Units, or YPG, are a key US ally in the fight against IS.
Washington regards them as the most effective fighting force on the ground in Syria and has provided weapons and special forces military advisers.
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