The leaders of Russia and Turkey held crisis talks on Friday to try to scale down tensions after 33 Turkish soldiers were killed in a regime air strike in Syria.
The international community voiced fears of a rapidly-rising risk of escalation after the attack by Russian-backed Syrian forces in the province of Idlib, where President Bashar al-Assad is waging a bloody campaign to oust rebels from their last holdout.
Hours after the strike, Turkey warned it was opening the gates for refugees to flee to Europe, a move that could have major repercussions for its western neighbours and prompted Greece to double its border patrols.
The deadly bombardment added to weeks of mounting tensions between rebel supporter and NATO member Ankara and Damascus ally Moscow, and stoked international concerns about the plight of people living in the battleground province.
It was the highest single death toll since the Turkish army first intervened in Syria in 2016, and brought the total number of Turkish troops killed in Idlib this month to 53.
Turkey said it retaliated to Thursday's deaths by hitting more than 200 regime targets in drone and artillery bombardments.
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The reprisals killed 16 Syrian soldiers, according to a monitoring group, but there was no immediate confirmation from Damascus.
Adding to the tensions, Moscow announced that two of its warships were transitting through the Bosphorus Strait in Istanbul in plain sight of the city.
But in a move to try to defuse the crisis, Russian President Vladimir Putin and his Turkish counterpart Recep Tayyip Erdogan spoke by phone on Friday.
"There is always room for dialogue," said Russian Foreign Minister Sergei Lavrov.
"The conversation was detailed and devoted to the necessity to do everything" to implement a ceasefire deal agreed in 2018 between the two countries to try to bring calm to Idlib.
Lavrov said Russia was ready to help improve the security of Turkish troops in Syria after the defence ministry said the slain soldiers had been among "terrorist groups" in Idlib.
The UN has repeatedly warned that the fighting in Idlib could potentially create the most serious humanitarian crisis since the start of the Syrian conflict in 2011.
But Russian vetoes, often backed by China, have chronically crippled UN action. NATO held urgent talks on the crisis on Friday, and joined the UN and the US in calling for urgent de-escalation.
"There is a risk of sliding into a major open international military confrontation," said EU foreign policy chief Josep Borrell, echoing international alarm about the violence.
"It is also causing unbearable humanitarian suffering and putting civilians in danger."