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.
- Extra patrols -
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Thursday's government raids were the first time the regime had bombarded Kurdish positions from the air.
As soon as the strikes began, Kurdish ground forces unsuccessfully tried to hail the pilots via radio.
US forces then contacted Russia, which has been bombing parts of Syria for nearly a year in support of 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 of the US-supported Kurdish People's Protection Units fighting the Islamic State group in northern Syria.
The coalition is now conducting additional combat air patrols in the region, he added.
The regime and Kurdish forces share a common enemy in IS, which controls most of the Euphrates valley to the south, but there have been tensions between them in Hasakeh that have sometimes led to clashes.
The Kurds, who control much of northeastern and northern Syria along the Turkish border where they have proclaimed an autonomous Kurdish region, recently demanded that the pro-government National Defense Forces disband in Hasakeh.
A government source in the city told AFP that the air strikes were "a message to the Kurds that they should stop this sort of demand that constitutes an affront to national sovereignty."
The 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 advisers.
Observatory chief Rami Abdel Rahman told AFP US special forces were based some six kilometers (nearly four miles) north of Hasakeh and that "new reinforcements" arrived Friday "from inside and outside Syria, accompanied by military helicopters."
Separately, two Russian ships in the Mediterranean launched long-range cruise missiles against jihadist targets in Syria, the Russian defense ministry said.
Russia's navy fired them Friday morning in its first use of cruise missiles against Syria since December.
It said two Buyan-class corvettes including its new Zelyony Dol patrol ship staged three launches of Kalibr missiles against targets linked to the former Al-Nusra Front group, which has renamed itself the Fateh al-Sham Front.
More than 290,000 people have been killed since Syria's conflict erupted in March 2011.