A Syrian cease-fire brokered by Russia and Turkey took effect early Friday but already appeared in doubt after one of the country’s biggest rebel groups rejected the deal just hours before it was to start.

The rebel group Ahrar al-Sham said late Thursday that it had unspecified reservations over the cease-fire, hammered out by Turkey and Russia as part of a road map to peace talks between the Syrian government and the opposition. Moscow and Ankara had lauded their Syria peace plan as the best solution to ending the war, part of warming relations between the capitals as they sideline the U.S. from negotiations.

Earlier in the day, Russia’s defense ministry had said Ahrar al-Sham was one of seven groups representing some of Syria’s strongest rebel factions that were included in the truce. The truce was intended to prompt smaller groups to join the cease-fire and lend momentum to the peace process.

Those prospects now appear dim without Ahrar al-Sham’s participation. Syria has seen two nationwide cease-fires this year, only to have them break down after violations by both rebels and government forces.

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Featured image courtesy of CNN.