The airstrikes took place on Thursday, conducted by two Syrian SU-24 attack planes targeting Kurdish forces undergoing training with US special operations advisers around the northeastern city of Hassakeh, Pentagon spokesman Captain Jeff Davis said.
The coalition scrambled its own jets to the area in a bid to intercept the Syrian jets, but the regime planes had left by the time they arrived.
"This was done as a measure to protect coalition forces," Davis said.
"We will ensure their safety and the Syrian regime would be well-advised not to do things that place them at risk," he added. "We view instances that place the coalition at risk with utmost seriousness and we do have the inherent right of self-defense."
But the warning appeared to fall on deaf ears. Two Syrian regime warplanes attempted to fly to the area again on Friday, but were met by coalition aircraft, a US defence official said in a statement.
"The presence of the coalition aircraft encouraged the Syrian aircraft to depart the airspace without further incident," he said, "No weapons were fired by the coalition fighters."
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.
"This is very unusual, we have not seen the regime take this type of action against YPG before," Davis said, using the initials of the US-supported Kurdish militia fighting the Islamic State group in northern Syria.
No coalition injuries were reported in Thursday's strike. US forces were 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 the IS group.|
The Kurdish People's Protection Units, or YPG, are a key US ally in the fight against the IS group.
Washington regards them as the most effective fighting force on the ground in Syria and has provided weapons and special forces military advisers.
But to Ankara the forces are a branch of the Kurdistan Workers' Party [PKK], designated a "terrorist" group by Turkey.
After Iran's Foreign Minister Mohammed Javad Zarif met with his Turkish counterpart Mevlut Cavusoglu earlier this month, Ankara reiterated its position on the YPG forces, with Cavusoglu linking them once more to Turkey's PKK and Iran's PJAK.
For the YPG, recent attacks on its forces near Hassakeh are directly linked to Turkey and Iran's position towards the group.
"The Syrian regime is apparently working with some regional countries and especially Turkey, which intervened directly in Syria and Rojava," the local self-administration of Syria's Kurdish region said in a statement on Friday.
"This is an indication of coordination between Turkey, Iran and the Baathist regime in Syria, to stop the implementation of the democratic project in northern Syria and Rojava," the administration said.
Syrian regime aircrafts bombed Kurdish positions for the first time on Thursday, killing at least five people and injuring dozens more.
YPG spokesman Redur Xelil said the attacks came only after Kurdish-led Syrian Democratic Forces [SDF] made advances against Islamic State [IS] group in Manbij.
"This happened after SDF and YPG have achieved victories against Daesh [IS] to empty the meaning of Manbij liberation," Xelil said, "Because they see the defeat of Daesh [IS] as their defeat."
Agencies contributed to this report.