BEIRUT (Reuters) - Syrian government warplanes bombed Kurdish-held areas of the northeastern city of Hasaka on Thursday for the first time in the country’s five-year-old civil war, killing at least 13 people, the Syrian Kurdish YPG militia and a monitoring group said.

The powerful YPG, a crucial partner in the U.S.-led war against Islamic State, said it would “not be silent” over what it called it an act of flagrant aggression. There was no immediate comment from the Syrian government.

People’s Protection Units (YPG) spokesman Redur Xelil said the air strikes had hit Kurdish districts of the city, which is mostly controlled by Kurdish groups, and positions held by a Kurdish security force known as the Asayish.

“There are martyrs and wounded,” he told Reuters.

Government forces were also bombarding Kurdish districts of Hasaka with artillery, and there were fierce clashes in the city.

“Every hand spattered with the blood of our people will be held to account through all possible and available means,” the YPG said in a statement.

The YPG and Syrian government forces have mostly left each other to their own devices in the conflict, during which Kurdish groups have exploited the collapse of state control to establish autonomy across much of the country’s north.

The government, which routinely uses its air force against rebels fighting Syrian President Bashar al-Assad in western Syria, still has footholds in the cities of Qamishli and Hasaka, both in the Hasaka governorate.

The Syrian Observatory for Human Rights, which tracks the war using a network of activists, said warplanes had targeted Kurdish security forces’ positions in the northwest and northeast of Hasaka city.

It said clashes were also taking place in a number of places around Hasaka.

At least thirteen people, including children and women, were killed as a result of shelling by the army on Kurdish-controlled areas in the city, the monitor said.

Syria’s complex, multi-sided war has created a patchwork of areas across the country controlled by the government, rebels, Kurdish forces or Islamic State.

The YPG makes up a significant portion of the U.S.-backed Syria Democratic Forces (SDF), a Kurdish and Arab alliance fighting Islamic State insurgents in Syria.

Last week the SDF, backed by air strikes from the U.S.-led anti-IS coalition, said they had ousted Islamic State from the city of Manbij near the Turkish border after a two-month campaign.

The recent fighting marks the most significant violence between the YPG and government forces since several days of fighting in Qamishli in April.