An airstrike on a rebel base in northwest Syria on Monday killed at least 35 Turkey-backed fighters, according to a spokesman and a war monitor, in what is believed to be the deadliest attack since a ceasefire in Idlib began almost eight months ago.
UK-based monitoring group the Syrian Observatory for Human Rights put the number of dead at 78.
Turkey and Russian Federation had brokered a truce in Idlib earlier this year to halt a government offensive that displaced hundreds of thousands.
The National Liberation Front (NLF), a coalition of Turkey-backed rebels, confirmed to AFP that Russian strikes had targeted one of its facilities but did not give an exact death toll.
Observatory head Rami Abdel Rahman described Monday's strikes near the Turkish border as the "deadliest since the ceasefire came into force".
Hayat Tahrir al-Sham, led by Syria's former al-Qaida affiliate, and allied rebels dominate the region of some three million people, around half living in camps after being displaced by fighting in other parts of the country.
In March, a truce brokered by Russian Federation and Turkey prevented pro-Assad forces from overrunning Idlib, the last remaining rebel-held area in northwest Syria, which holds at least a million Syrian refugees who have been displaced from across the country.
A spokesman for the NFL, Naji al-Mustafa, said the rebels' military retaliation targeted and killed Russian officers in southern Idlib, as well as Syrian soldiers working in the area. But the truce remained shaky. Around a dozen Turkish observation points were deployed inside Idlib to monitor the truce.
On Friday, airstrikes also targeted a local market for rudimentary fuel burners and diesel in the opposition-controlled region of Jarablus, in northern Aleppo. Syrian opposition fighters said it was part of Turkey's redeployment of its forces in the shrinking enclave.
Syria is witnessing a bloody conflict that, since its outbreak more than nine years ago, has caused the death of more than 380,000 people and widespread destruction of the infrastructure, in addition to the displacement and displacement of millions inside and outside Syria.
The outpost in Morek had been Turkey's largest in Hama province, most of which is now under Syrian government control.
Last week, Turkish troops evacuated one of their largest military bases in the area, which had been surrounded by Syrian government troops for months.
Endless rounds of UN-backed peace talks have failed to stem the bloodshed and in recent years have been overtaken by a parallel negotiations track led by Russian Federation and Turkey.
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