Syrian opposition groups welcomed the US-Russian ceasefire deal on Sunday, writing a letter to the US and saying they are ready to cooperate positively even if they have major reservations about its terms.
The Free Syrian Army who wrote the letter believe that the deal due to take effect on Monday evening, would back the Syrian regime and treat them unfairly.
"A big part of the agreement serves the regime and doesn't apply pressure on it and doesn't serve the Syrian people," said Zakaria Malahifji of the Aleppo-based armed group Fastaqim.
The groups are also worried about the absence of enforcement mechanisms, a lack of provision for besieged areas and clauses letting army jets fly for up to nine days after the deal comes into effect.
The influential armed group Ahrar al-Sham issued a statement late on Sunday attacking the ceasefire deal, but stopping short of explicitly saying it would not abide by its terms.
Ahrar al-Sham which works closely with former Al-Qaeda affiliate Fateh al-Sham Front, said in a statement that the deal would only serve to "reinforce" the Assad regime and "increase the suffering" of civilians.
"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.
"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 added.
The ceasefire will not apply to the groups such as DAESH or Jabhat Fateh al-Sham.
Fateh al-Sham group - known as the Nusra Front until it broke formal allegiance to al Qaeda and changed its name - 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."
Syria's five-year civil war has killed hundreds of thousands of people and displaced 11 million - half the country's pre war population - causing a refugee crisis in the Middle East and Europe.
Syrian regime has not issued an official comment on the truce, but Syrian state media on Saturday quoted what it called private sources as saying the regime had given its approval. Iran welcomed the deal on Sunday.
More than 45 people have died following the intense airstrikes in the Syrian cities of Aleppo and Idlib on Saturday hours after the US and Russia announced plans for the ceasefire.