"Concerning the forthcoming meeting in Astana, the (High Negotiations) Committee stresses its support to the military delegation... and expresses hope that the meeting would reinforce the truce," an HNC statement said after a two-day meeting in Riyadh.
Despite backing opposite sides in the Syrian conflict, Russia and Turkey have worked closely in recent weeks to broker a nationwide ceasefire aimed at laying the ground for the January 23 peace talks in Astana.
It is not yet clear which groups have been invited to attend.
Russian foreign ministry spokeswoman Maria Zakharova has said on Thursday there was "no precise information" on participants for now.
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The HNC expressed hope that the talks would "establish a phase of confidence" through the implementation of UN Security Council Resolution 2254, especially articles concerning ending sieges of cities and towns, delivering aid and releasing detainees.
The HNC said it "appreciates efforts" to make the Astana talks fruitful, adding that the meeting represents a step that "paves the way for political talks" in Geneva next month.
The statement stressed that "discussing the political track... should be held under the UN sponsorship and supervision."
The HNC is the main Syrian opposition umbrella group and participated in previous peace talks in Geneva.
The ceasefire and planned talks are the latest effort to negotiate an end to a conflict that has killed more than 310,000 people since it began with anti-government protests in March 2011.
Moscow and Ankara brokered a deal that allowed civilians and rebels to leave the opposition enclave in Aleppo before regime troops took full control of the city last month.