Russia and Turkey are in intense negotiations to ensure the rebel-held province of Idlib does not become a breaking point in their alliance on Syria, but the long term fate of the area still risks provoking a rupture, analysts say.
President Vladimir Putin and Turkish counterpart Recep Tayyip Erdogan have spearheaded an unlikely but so far sustained partnership to bring peace to Syria since late 2016, despite being in theory on opposite sides of the civil war.
The cooperation now faces its biggest test over Syria's northwestern province of Idlib, bordering Turkey, which Syrian President Bashar al-Assad wants to recapture to complete a string of military successes.
Idlib has over the last years become an increasingly densely-populated region where problems in resolving the seven-year Syrian civil war were effectively dumped to be resolved at a later date.
That moment is now drawing closer with expectations of a government offensive and fears over the combustible mix in the province of displaced people from other Syrian regions, moderate rebels and Islamist radicals.
Erdogan has been a champion of the anti-Assad rebels and bitterly denounced Putin in the crisis that followed the shooting down of a Russian war plane by Turkey in November 2015.
But the burgeoning alliance with Moscow, which also includes cooperation in trade, energy and defence, is now of critical importance to Ankara at a time when a crisis in relations with the US has caused the Turkish lira to bleed value.
"There is still a hope in Moscow to find an agreement with Ankara that could allow the Syrian regime to take control of Idlib without a new rupture with Turkey," Kerim Has, a Moscow-based analyst on Turkish-Russian relations, told AFP.
He said Moscow was carefully watching Turkey-US ties and could launch a full operation at the "moment when the Turkish leadership is in most desperate need of the Kremlin's encouragement during a deepening Turkey-US crisis."
- 'Limited offensive' -
Turkish Foreign Minister Mevlut Cavusoglu on Friday led a delegation including Defence Minister Hulusi Akar and spy chief Hakan Fidan to Moscow to discuss the Idlib situation and also Moscow's planned delivery of S-400 missile systems to Turkey.
Cavusoglu warned that a "military solution" in Idlib, which he said is now home to 3.5 million people, would be a "catastrophe" and provoke a new refugee influx into Turkey where three million Syrians already live.
But he also said "radical groups and terrorists" in Idlib should be "neutralised", in comments seen as indicating Ankara could support a limited intervention.
Russian Foreign Minister Sergei Lavrov said a differentiation should be made between moderate rebels and extremists, like the Hayat Tahrir al-Sham group (HTS) who control much of Idlib.
In a sign of the importance of the visit, Cavusoglu was received by President Putin, who praised the "deep cooperation" over Syria. Cavusoglu told Putin "your dear friend Mr Erdogan" expected him in Istanbul soon for dinner at a fish restaurant.
"Russia and Turkey are trying to find agreement with suitable terms for both sides," Timur Akhmetov, Ankara-based researcher at the Russian International Affairs Council, told AFP.
He predicted Russia and the Syrian regime would launch a "limited offensive" against groups linked to former Al-Qaeda affiliate Al-Nusra Front -- especially on factions that have irritated Moscow by launching swarm attacks with drones against its Hmeimim base, the heart of Russian operations in Syria.
"Russia is trying to encourage Turkey to have the opposition contribute to the offensive and in return, these groups will be spared from air attacks," Akhmetov added.
- 'Direct opposition' -
Yet as has been the case repeatedly for Idlib, a limited offensive will only temporarily sidestep the key issue, namely who will control the province long-term and eventually postwar.
And even if Russia can agree a compromise, it is not always the case that Assad automatically follows Moscow's line or that his other main ally Iran backs the Kremlin.
"The Russians and the Turks will be hard pressed to reach a sustainable, long-term agreement acceptable to the regime in northwestern Syria," Elizabeth Teoman, Turkey analyst for the Institute for the Study of War (ISW), told AFP.
Turkey has set up 12 so-called observation points in Idlib staffed by its military, ostensibly aimed at monitoring a de-escalation zone but which also serve to protect Ankara's interests.
Teoman said Erdogan could contemplate a possible limited offensive -- with Russian and Turkish interests overlapping for now on limiting a refugee influx and promoting reconstruction in Idlib.
She said the current discussions likely did not represent an "imminent breaking point" between Moscow and Ankara but warned: "Turkey's aspirations are in direct opposition to Assad's goals to crush the remnants of the Syrian rebellion."