An influential Syrian rebel group, the hardline Islamist Ahrar al-Sham, on Sunday rejected the truce deal brokered by Russia and the United States hours before it was due to begin.
A high-ranking member of the group, which works closely with former Al-Qaeda affiliate Fateh al-Sham Front, said in a statement on YouTube that the deal would only serve to "reinforce" the regime of President Bashar al-Assad and "increase the suffering" of civilians.
Ahrar al-Sham, which has an influential presence on the ground in Syria's brutal war, is the first rebel group to officially react to the deal reached on Friday following marathon talks in Geneva.
"The people cannot accept half-solutions," the group's deputy leader Ali al-Omar said in the YouTube video to mark the start of the Muslim holiday of Eid al-Adha on Monday, when the truce is due to come into force.
"The Russian-American deal... will send all the sacrifices and gains of our people who have risen up into smoke. It will only serve to reinforce the regime and surround the revolution militarily."
Omar also rejected the aspect of the deal in which Washington is supposed to convince mainstream opposition fighters to break their alliance with Fateh al-Sham.
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That group, which is not covered by the truce deal, has also dismissed the accord.
"It's simple -- the Russian-American deal is intended to eliminate those who protect Syrians," Fateh al-Sham spokesman Mostafa Mahamed wrote on Twitter.
"The negotiations and the deals which do not take account of fighters on the ground are useless."
The announcement of the truce after weeks of talks between the US and Russia -- which back opposing sides in the Syrian war -- comes with opposition forces struggling on the ground.
Pro-regime forces reimposed a devastating siege on Aleppo's eastern districts last week in a major blow to the rebels.
The Assad regime and its allies have said they will respect the truce deal.