Ahrar al-Sham claimed the US-Russia brokered deal would have little political influence as it only empowers the Assad government
The breakthrough ceasefire deal touted as the moment of change in the Syrian peace process got off to a shaky start.
Russia and the United States hammered out a truce deal following marathon talks on Friday in the Swiss city of Geneva.
But just hours before the ceasefire was due to begin, an influential Syrian rebel group, the hardline Islamist Ahrar al-Sham rejected the truce deal.
The Syrian rebel group is the first to officially react to the deal. Ahrar al-Sham said the people cannot accept half-solutions.
In his YouTube video to mark the start of the Muslim holiday of Eid al-Adha on Monday, the group's deputy leader Ali al-Omar said that the deal would only serve to reinforce the regime of President Bashar al-Assad and increase the suffering of civilians.
The group works closely with former Al-Qaeda affiliate Fateh al-Sham Front, which has also dismissed the accord.
The deputy leader refuted a crucial aspect of the deal wherein Washington is expected to convince mainstream opposition fighters to break their alliance with Fateh al-Sham.
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 group is not covered by the ceasefire deal.
The negotiated plan aims to put Syria's peace process back on track. It will include a nationwide truce effective from sundown on Monday, providing better humanitarian aid access and military coordination between the U.S. and Russia against banned Islamist groups in Syria.
US secretary of state John Kerry said, ''The United States and Russia are announcing a plan which we hope will reduce violence, ease suffering and resume movement towards a negotiated peace and a political transition in Syria and we believe that the plan as it is set forth if implemented, if followed, has the ability to provide a turning point, a moment of change.''
The Assad regime and its allies have promised to respect the ceasefire deal.
Kerry said the "bedrock" of the new deal was an agreement that the Syrian government would not fly combat missions in an agreed area on the pretext of hunting fighters from the banned Nusra Front, an Al-Qaeda affiliate in Syria.
In another confidence building measure, Russian-backed government forces and opposition groups, supported by the United States and Gulf States, would halt fighting for a while. It would give a chance to the opposition fighters to separate from militant groups in areas, such as Aleppo.
(WION with inputs from agencies)