Turkey on Sunday launched airstrikes against a United States-backed Kurdish militia in the autonomous Afrin province in Syria, Reuters reported. The operation, named “Operation Olive Branch”, hit 108 targets of the Syrian-Kurdish People’s Protection Units militia, or YPG, the agency said quoting Turkish military officials.

“The weakening of the region with artillery fire is under way,” Turkish Prime Minister Binali Yildirim said. “The first stage was carried out by aerial forces of the military and nearly all of the targets were destroyed.”

Troops have already crossed into northern Syria, Yildirim was quoted as saying by the BBC. He said the aim was to establish a 30-km “safe zone” deep inside Syria, local media reported.

The Turkey-backed Free Syrian Army rebels also helped the operation. Yildirim said they would also carry out “necessary activities” depending on the situation. He did not elaborate.

The YPG said that the strikes killed six civilians and three fighters. One of the fighters belonged to the YPG and two others were from its all-female YPJ – the Women’s Protection Units. “We will defeat this aggression, like we have defeated other such assaults,” the YPG said in a statement. The organisation said that 13 civilians were also injured.

Turkey’s action might worsen its ties with the United States, Reuters said. The US supports the Kurdish YPG, as a partner in the fight against the Islamic State in Syria.