Turkey has raised the stakes in Syria by unleashing a campaign of heavy cross-border shelling against Russian-supported fighters from the People's Protection Units (YPG) that Ankara insists will continue despite international calls to desist. Ankara wants to prevent the Kurdish group, which it considers a terrorist organization allied with the Kurdistan Workers Party, from gaining more territory along Turkey’s border with Syria. In a separate development bound to further increase tensions, Turkey also shelled Syrian military positions across the border from Hatay province around the same time it unleashed its campaign against the YPG.

Turkish military sources told the daily Hurriyet that the shelling of Syrian army and YPG positions was in response to mortar attacks against Turkish territory. While this might be true with regard to the Syrian army, remarks by Prime Minister Ahmet Davutoglu indicate that Turkey has broader goals when it comes to the YPG.

A complication for Ankara, however, is that both the United States and Russia are supporting the YPG, albeit for different reasons. YPG fighters have advanced against the Islamic State (IS) east of the Euphrates River with US air support, while Russia has been using them against anti-Assad forces west of the Euphrates.

Meanwhile, animosity between Ankara and Moscow has been deepening since Turkey downed a Russian jet fighter Nov. 24. In addition, Washington is unhappy with the YPG’s Russian-supported advances, but nonetheless greeted Turkey’s military response against the group with disapproval.