More than 290,000 people have been killed since Syria's conflict erupted in 2011.
The US-led coalition scrambled fighters after Syrian regime jets targeted Kurdish forces working with US advisers in northern Syrian, an incident marking another escalation in Syria's bloody conflict, the Pentagon said today.
Yesterday'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.
"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... 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 as Syrian regime warplanes bombarded Hasakeh for a second day today.
As soon as yesterday's strike began, ground forces tried to hail pilots via radio, but 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.
By the time US planes had arrived in the skies over Hasakeh, the Syrian planes had already left the area. No coalition injuries were reported in yesterday's strike.
The coalition is now conducting additional combat air patrols in the region, Davis said.
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.
More than 290,000 people have been killed since Syria's conflict erupted in March 2011.