Turkish-Russian relations are entering a new stage as they prepare to start joint patrols of a strip 10 kilometers (six miles) deep inside northeastern Syria Friday following Ankara’s incursion into the region this month to expel US-backed Kurdish forces.

As the two nations coordinate operations on the ground, negotiations are taking place between state officials over Ankara’s possible purchase of Russian-made Su-35 warplanes. Turkish President Recep Erdogan expressed interest in the jets following Turkey’s acquisition of Russian S-400 missile defense systems in July, which riled long-time NATO allies and may result with sanctions for violating regulations meant to deter purchases of Russian military technology.

While such sanctions have yet to be imposed on Turkey — partly due to resistance within the White House — the S-400 purchase did lead to Ankara’s expulsion from the Western bloc’s F-35 fight jet program. Though the Turkish state has already spent millions on the production and development of next-generation F-35 warplanes and the nation’s economy is expected to lose $9 billion due to its exclusion, NATO officials claim the move was necessary to safeguard the F-35's security features, which could be compromised if the jets were operated within the range of S-400 radars.

Left with few options to modernize Turkey’s aging fleet of F-16s, Ankara officials are now considering Russia’s Su-35 jet as a viable option, though its acquisition would risk further damaging Turkey’s already troubled relations with Western and NATO allies.