Warplanes attacked Turkey-backed opposition fighters in northern Syria on Sunday, killing and wounding about 20, an opposition war monitor and pro-government media said.

The airstrike struck a position in an area near the town of Afrin. It came amid increasing tensions between government forces and insurgent groups who still have a stronghold in northwestern Syria, mainly in the province of Idlib.

The Sunday morning airstrike was carried out by Russian warplanes, opposition activists said.

The airstrike hit a center for a Turkey-backed group known as the Hamza Division killing seven fighters and wounding 13, according to the Britain-based Syrian Observatory for Human Rights.

Pro-government media also reported Russian airstrikes near Afrin saying that a number of gunmen were killed or wounded.

Russia joined the war in September 2015 helping tip the balance of power in favor of President Bashar Assad’s forces while Turkey has been a main backer of the opposition.

Syria’s conflict that broke out in March 2011 has left hundreds of thousands dead and displaced half the country’s pre-war population of 23 million, including more than 5 million refugees outside the war-torn nation.

On Friday, United Nations human rights chief Michelle Bachelet said her office has documented the deaths of 350,209 people — civilians and combatants — in Syria’s civil war. She said the real number is almost certainly far higher.

Associated Press