On Saturday the Turkish Armed Forces opened fire on fighters from the Kurdish-dominated People's Protection Units (YPG) and Jaysh al-Thuwar, an Arab majority militia, near Syria's border town of Azaz. These two groups have taken advantage of the recent Russian and regime-led offensive north of Aleppo to challenge the rebel-held towns of Tel Rifaat and Azaz.
Turkey's actions suggest that it is prepared to defend the town from the YPG, lest it otherwise risk the town falling to groups Ankara deems as hostile. The YPG is the Syrian affiliate of the Kurdistan Workers' Party (PKK), a Turkish and United States-designated terror group.
Apart from by a Turkish ground incursion, or deft diplomacy, it is unlikely that Turkey can stem the YPG's advance in the Azaz corridor. The YPG has managed to secure support from the two most powerful external actors in the Syrian conflict: the Russian Federation and the US.
The Kurdish factor
The US is currently working with the YPG to put pressure on Raqqa, the Islamic State of Iraq and the Levant (ISIL) group's existing capital in eastern Syria. Russia, in contrast, is using the Syrian Kurds to put pressure on Turkey and fracture the anti-ISIL coalition. The Russian strategy, in turn, would empower Syrian President Bashar al-Assad's bargaining power at the concurrent peace talks in Geneva.
The YPG's opportunistic exploitation of these two alliances serves its immediate interest: Connecting its territory in Afrin with Kobane and Jazira in the west of the Euphrates. However, the tolerance of the regime towards the YPG makes the possibility of rapprochement with Arab groups in the area extremely difficult - if not impossible. Turkey has sought to exploit these cleavages and prevent elements within the anti-Assad opposition from co-operating with the YPG in Azaz and Aleppo.
Turkey's recent shelling of Kurdish forces suggests that the war in Syria is transitioning to a new phase.
In the short term, the Russian bombardment of northern Aleppo has opened a viable route for YPG - and its associated political party, the Kurdish-majority Democratic Union Party (PYD) - to take Tel Rifaat.
In the east of Aleppo, Assad's forces appear poised to take the town of al-Bab from ISIL, which would open a corridor for YPG-backed forces to occupy the west bank of the Euphrates, near the Tishreen Dam, and help them to advance towards Manbij. And, if the regime continues north and takes al-Bab, the YPG can reach an agreement with Assad's forces for freedom of movement between Afrin and Kobane via Aleppo.
This short-term success, however, portends longer-term problems in Sunni Arab-dominated territory in Syria, east of Azaz and north of Manbij. Much of this territory is now under the control of ISIL. A smattering of anti-Assad forces, including the Ahrar al-Sham, is deployed along the so-called Marea line. This defensive position protects the Azaz corridor from ISIL encroachment - and the US conducts air strikes on these groups' behalf. The YPG's forward advance from the west places the group on a collision course with ISIL.
To be sure, in such a scenario, the YPG would be attacking from the east and west. However, it would also have to contend with the Arab groups active in the area. The Kurds would thus be involved in a multipronged fight, where, again, its partner of convenience would be the Assad regime and the Russian Air Force. This is not a long-term model for stability. Instead, it is a recipe for perpetual conflict with Arab groups backed by Turkey and other regional powers.
Such an advance is not in the immediate interest of the US; Washington has put pressure on the YPG to cease its co-operation with the Russian Air Force near Afrin and to reach an accommodation with Sunni Arab groups in the region.
OPINION: Syrian Kurds and Turkey's Kurdish question
The US has also pushed the YPG to halt its offensive on the eastern bank of the Euphrates, rather than continue towards Manbij. This policy is, largely, a result of US deference to Turkey's concerns about the Syrian Kurds controlling its longest land border.
The Russian intervention changed the game. Turkey is quickly running out of options in the Azaz corridor and, in eastern Syria, recent developments suggest that the US will continue its push towards Raqqa.
The United Arab Emirates (UAE) will deploy special operations forces to train Sunni fighters for this mission. Additionally, along with Saudi and Qatar, the country is deploying jets at Incirlik air force base in Turkey, as part of the US-led anti-ISIL coalition.
These jets are certain to be integrated into the coalition's air-tasking order, which tracks, co-ordinates and deconflicts all coalition air operations. This suggests that these fighters will bolster the current U.S. strategy: the defeat of ISIL. These two events, therefore, appear to be separate from one another.
Turkey's recent shelling of Kurdish forces suggests that the war in Syria is transitioning to a new phase. In the Azaz corridor, Turkey has pledged to continue attacking the YPG. At the same time, the coalition will continue its air operations along the Marea line and, more importantly, in support of the YPG east of the Euphrates.
These aspects of the war are moving in parallel to the Assad regime's Russian-supported effort to surround Aleppo and move north to al-Bab. Further still, in Idlib, a different subset of the anti-Assad opposition is also battling the regime, including Syria's al Qaeda affiliate, al Nusra Front, a group the US has pledged to combat.
The dynamics of the current conflict point towards perpetual conflict, regardless of progress made towards the defeat of ISIL. The actors involved - both at the regional and sub-state level - remain hostile to one another.
Aaron Stein is a senior fellow at the Atlantic Council's Rafik Hariri Center for the Middle East.
The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.
Source: Al Jazeera