Turkish President Recep Tayyip Erdogan will host his Russian and Iranian counterparts on Monday for their latest summit on Syria, with attention focused on Damascus's push on the last rebel-held bastion of Idlib.
Presidents Vladimir Putin and Hassan Rouhani will join Erdogan in the Turkish capital Ankara for their fifth summit on the conflict since 2017.
Iran and Russia have been staunch supporters of Syrian President Bashar al-Assad, while Turkey has called for his ouster and backed opposition fighters.
But with Assad's position looking increasingly secure, Turkey's priority has shifted to preventing a mass influx of refugees from Idlib in Syria's northwest.
Turkey is concerned by the steady advance of Syrian forces into the region, backed by Russian airpower, despite a series of ceasefires.
Turkey has 12 observation posts in Idlib to enforce a buffer zone agreement struck a year ago with Russia to prevent a full-scale Syrian offensive.
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But the posts look increasingly threatened, with one of them cut off from the rest of Idlib when Syrian forces advanced last month.
Russian air strikes have continued in the region despite the latest ceasefire between Ankara and Moscow on August 31.
"A large number of terrorists are still present in this zone... and fighters continue to fire on the positions of government forces," Kremlin advisor Yuri Ushakov said on Friday.
The Turkish presidency said the leaders would discuss the latest developments in Syria as well as "ensuring the necessary conditions for the voluntary return of refugees and discussing the joint step to be taken in the period ahead with the aim of achieving a lasting political solution."