A Turkish air strike on Monday killed at least 11 Kurdish security personnel and wounded dozens at a training center for police in Kurdish-held northeast Syria, according to the Syrian Observatory for Human Rights and Kurdish authorities.

The strike is the latest in a series of attacks by Turkey in the region, which has been under the control of Kurdish forces since 2012. 

Turkey views the Kurdish People’s Protection Units that dominate the Kurdish-led Syrian Democratic Forces as an offshoot of the Kurdistan Workers’ Party, which it considers a terrorist group.

The YPG has been a key US ally in the fight against ISIS in Syria, but Turkey has repeatedly called on the US to withdraw its support for the group.

The Turkish defense ministry said the strike was in retaliation for an attack in Ankara earlier this month that wounded two security personnel. A branch of the PKK claimed responsibility for that attack.

The US has urged Turkey to show restraint in its attacks in Syria and has expressed concern about the potential impact on the fight against ISIS.

The Syrian civil war has been raging for over a decade, killing hundreds of thousands and displacing millions more. The conflict has also created a power vacuum exploited by extremist groups such as ISIS.