The fighting this week in Hasaka marks the most violent confrontation between the Kurdish YPG militia and Damascus.

Civilians fled a city in northeastern Syria where government warplanes bombed Kurdish-held areas for a second day on Friday, as the Syrian army accused Kurdish forces of igniting the conflict by trying to take over the area.

The fighting this week in Hasaka, which is divided into zones of Kurdish and Syrian government control, marks the most violent confrontation between the Kurdish YPG militia and Damascus in more than five years of civil war.

The YPG is at the heart of a US-led campaign against ISIS in Syria, and controls swathes of the north where Kurdish groups have set up their own government since the Syrian war began in 2011.

A Pentagon official said US-led coalition aircraft were sent near Hasaka on Thursday to protect coalition special operation ground forces in response to bombing by Syrian jets, and additional combat air patrols were being sent to the area.

The government air strikes on Hasaka mark the first time the Syrian military has deployed its warplanes against Kurdish groups during the war.

Pentagon spokesman Captain Jeff Davis said the incident was not an intercept and the coalition aircraft reached the area by the time the Syrian government warplanes were leaving.

YPG spokesman Redur Xelil told Reuters that Kurdish authorities had evacuated thousands of civilians from their area of control. “Whoever can bear arms is fighting the regime and its gangs,” Xelil said. “Our situation is so far defensive but it will change all the while the regime escalates in this way.”

In its first comment on the situation, the Syrian army accused a YPG-affiliated security force known as the Asayish of igniting the violence through escalating “provocations” including the bombardment of army positions in Hasaka that had killed a number of soldiers and civilians.

In a statement, the military command said the Asayish were aiming to take control of Hasaka city, and the army had responded appropriately by firing at armed groups’ “sources of fire”.

The Syrian Observatory for Human Rights, which tracks the war using a network of activists, said civilians were using lulls in the fighting to flee the city. Xelil said dozens of civilians had been killed over the past two days.

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.

Assad, backed by Russia and Iran, has focused mostly on fighting Sunni Arab rebels who have been battling to oust him in western Syria with support from countries including Turkey, Saudi Arabia and the United States. The Syrian and Russian air forces are routinely deployed in the war in western Syria.

The YPG, or People’s Protection Units, has meanwhile prioritized carving out and safeguarding predominantly Kurdish regions of northern Syria. The group has ties to the Kurdistan Workers Party (PKK) in Turkey.

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

It is the second major eruption of fighting between the YPG and Syrian government fighters this year. In April, they fought several days of lethal battles in Qamishli, north of Hasaka city and also mostly YPG-held.

The Observatory said Kurdish forces had gained ground in the southern part of Hasaka.

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

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