Russian air strikes in northern Syria have killed more than 50 Turkish-backed militia fighters in the mainly rebel-held province of Idlib, reports say.
Many others were wounded in the attack, which marks an escalation of violence in the region.
A training base for an Islamist group called Faylaq al-Sham was hit.
The assault puts at risk a ceasefire in Idlib, brokered and monitored by Russia and Turkey, which back opposite sides in the war.
UK-based monitoring group the Syrian Observatory for Human Rights put the number of dead at 78.
Some of the wounded were in a serious condition, and the death toll was likely to rise, the Observatory added.
It described the attack - in the Harem region north-west of Idlib city - as the deadliest since the ceasefire came into force in March.
The truce brought to a halt a Syrian government offensive on the region which had displaced almost a million people, and has largely held since then.
When the ceasefire was announced, Turkey said it reserved the right to "retaliate with all its strength" against any attack by forces allied to Syrian President Bashar al-Assad.
Idlib is the last province held by rebels and jihadists, who have been beaten back in a nine-year-long civil war.