Syrian government jets have continued to pound Kurdish-controlled parts of the northeastern city of Hasakah for a second day, killing at least 22 residents and forcing thousands to flee.

The Syrian Observatory for Human Rights (SOHR), which reports daily on the war using a network of activists, said on Friday that at least 22 civilians, including nine children, had been killed in the past two days.

Thousands of civilians, mostly women and children, were evacuated from the divided city on Friday, said Redur Xelil, a spokesman for the Kurdish People's Protection Units (YPG).

"Whoever can bear arms is fighting the regime and its gangs," Xelil told the Reuters news agency, adding that dozens had been killed in the air raids.

"Our situation is so far defensive but it will change while the regime escalates in this way," he said.

US precaution

A US-led coalition sent aircraft to Hasakah city on Thursday to protect American special operation ground forces from attacks by Syrian government jets, a Pentagon official said on Friday.

Captain Jeff Davis, the Pentagon spokesman, said the coalition aircraft reached the area around the city as the two Syrian SU-24s were leaving.

He said the Syrian planes did not respond to efforts by ground forces to contact them.

"We will ensure their safety and the Syrian regime would be advised not to do things that would place them at risk," Davis said.

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The air raids on Hasakah, which is divided into zones of Kurdish and Syrian government control, marks the most violent confrontation between the Kurdish YPG and Damascus in more than five years of civil war.

The YPG and Syrian government have mostly avoided confrontation during the multi-sided war that has turned Syria into a patchwork of areas held by the state and an array of armed factions.

While forces loyal to President Bashar al-Assad, backed by Russia and Iran, have focused mostly on fighting Sunni Arab rebels, the YPG has prioritised carving out and safeguarding predominantly Kurdish regions of northern Syria.

The group has ties to the Kurdistan Workers Party (PKK) in Turkey.

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The YPG controls most of the northeast, though the Syrian government has maintained footholds in the cities of Hasakah and Qamishli at the border with Turkey. The YPG has controlled most of Hasakah city since last year.

Rami Abdulrahman, of the SOHR, said the fighting began this week after pro-government militiamen detained a number of Kurdish youths, a step that was followed by advances of Kurdish security forces towards government-held areas.

Naser Haj Mansour, a Kurdish official in the YPG-affiliated Syria Democratic Forces alliance, said Kurdish forces had taken some additional positions, including an economics college.

The Syrian army said in a statement on Friday that the air raids were the result of Kurdish forces trying to take over the city.

The response was "appropriate", and any further such attacks would also be met with force, the army said in the statement, according to Reuters.