Syrian regime jets pounded US-backed Kurdish forces in northeastern Syria for a second day, even after the US-led coalition scrambled jets to protect its military advisers working on the ground.
In another escalation of Syria's bloody conflict, warplanes from President Bashar al-Assad's regime were bombarding the city of Hasakeh -- targeting Kurdish forces that for months have worked closely with coalition military advisers helping local fighters combat the Islamic State group.
On Thursday, the United States sent fighter jets to head off air strikes conducted by regime planes and to protect coalition advisers, but the Syrian planes had left by the time they arrived.
It was apparently the first time the coalition had scrambled jets in response to a regime action, and possibly the closest call yet in terms of Syrian forces coming close to killing American or coalition advisers.
"This was done as a measure to protect coalition forces," Pentagon spokesman Captain Jeff 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 the utmost seriousness and we do have the inherent right of self-defense."
But the Pentagon 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 defense 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."
Most of Hasakeh city is controlled by Kurdish forces, while the rest is held by fighters loyal to President Bashar al-Assad.
Since Wednesday, clashes between the two forces have rocked the city, leaving 23 civilians -- including nine children -- and 16 combatants dead, according to the Syrian Observatory for Human Rights.
Davis said no coalition injuries were reported in Thursday's strike by two Syrian SU-24s, and US special operations advisers have been moved to a safe location.
A Syrian military statement said Kurdish forces had surrounded Hasakeh and caused civilian and military casualties by shelling, and that it had taken the "appropriate response," targeting "the source of fire and positions of those responsible for these crimes."
The Observatory said thousands of inhabitants had begun to flee Hasakeh, where bread was running out and electricity supplies have been cut.