Syrian regime warplanes bombarded the northeastern city of Hasakeh for the second day on Friday, targeting positions held by Kurdish forces, a monitor and a journalist in the city said.

Fresh strikes came as the Pentagon said that US-led coalition fighter jets scrambled to protect Kurdish forces working with American advisers in Syria as they were targeted by regime planes Thursday.

The Syrian Observatory for Human Rights -- which monitors the conflict and determines which planes carry out raids according to their type, location, flight patterns and the munitions involved -- said Friday's strikes were conducted by regime jets.

The journalist in Hasakeh said eight air strikes could be heard throughout the day, mostly focused in the city's southwestern neighbourhoods.

Most of Hasakeh city, the capital of the northeastern province by the same name, 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 combattants dead, the Observatory said.

The Britain-based monitor said thousands of inhabitants had begun to flee Hasakeh, where bread was running out and electricity supplies have been cut.

Thursday's government raids on Hasakeh were the first time the regime had bombarded Kurdish positions from the air.

The development prompted jets from the US-led coalition battling the Islamic State group in Syria and Iraq to intervene in order to protect Kurdish forces working alongside US advisors, a Pentagon spokesman said Friday.

"This was done as a measure to protect coalition forces," Captain Jeff Davis said. "We did make clear that US aircraft would defend troops on the ground if threatened."

The Kurdish People's Protection Units (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.

IS controls most of the Euphrates valley to the south of Hasakeh, and tensions between regime and Kurdish forces have sometimes led to armed clashes in spite of their common jihadist enemy.

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 Defence 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".

More than 290,000 people have been killed since Syria's conflict erupted in March 2011.