Turkey issued its harshest warning yet to Russia in their standoff over Syria’s last rebel stronghold, where their battle for control threatens to destroy a fragile understanding that’s kept a refugee crisis from exploding at full force.

Turkish President Recep Tayyip Erdogan said his military “has made all the preparations to implement its own operation plans” in Idlib province in northwest Syria, which is controlled by a mix of onetime al-Qaeda militants and Turkish-backed rebels.

“An Idlib operation is just a matter of time,” Erdogan said in Ankara. “We won’t leave Idlib to those who haven’t understood our resolve, to the regime and those who encourage it.”

Turkey is trying to halt a Russian-backed Syrian government advance that’s threatening its efforts to establish a zone of control in Idlib. Turkey deployed thousands of troops this month to the province, where Syrian forces backed by Russian aircraft have besieged Turkish observation posts set up under accords meant to contain fighting, killing 14 Turks.

Read more: Bloomberg