"The HNC has not taken a decision yet," Monzer Makhos said.
"We are waiting for progress on the humanitarian issue and respect for the ceasefire. What has happened so far is not enough for us to participate."
UN envoy Staffan de Mistura told the pan-Arab newspaper Al-Hayat that the peace talks would start on Thursday.
"I think that we will begin on March 10. That is when the process will start," he said according to an Arabic translation of his remarks published by the newspaper.
While some delegates are expected to arrive in Geneva on March 9, others are not expected until March 11 or even 14 because of "problems with hotel reservations", De Mistura said.
He said preparatory meetings will be held ahead of "in-depth discussions separately" which each faction.
- Fate of Assad -
Since the failure of a first round of peace talks in 2014, the main sticking point in the negotiations has been the fate of Assad.
The Syrian president has refused to step down since peaceful protests in early 2011 developed into a multi-faceted war that has killed more than 270,000 people and displaced millions of people.
Al-Hayat reported De Mistura as saying a transition process would include "first, talks on a new government, second a new constitution and third parliamentary and presidential polls within the next 18 months".
The envoy said on Friday that the Syrian people, not foreigners, should decide Assad's fate.
But key opposition backer Riyadh on Saturday called for Assad -- whose clan has ruled Syria for more than half a century -- to step down at the start of any transition.
"Assad has to leave at the beginning of the process," Saudi Foreign Minister Adel al-Jubeir told reporters in Paris.
On the sequence of events, Jubeir said: "There is a transitional body, power shifts from Assad to the transitional body, and then he goes."
After that "the transitional body drafts a constitution, prepares for elections. Some are arguing that no, Bashar leaves at the elections in 18 months, that's not how we think."
Peace talks in early February were cut short amid intensifying Russian air strikes in Syria in support of Assad's forces.
A regime advance supported by Russian warplanes inflicted serious setbacks on the rebels and weakened the opposition's position in negotiations.
However, a fragile ceasefire drawn up by Russia and the United States with UN Security Council backing is now in its second week, despite accusations of violations.
- Opposition demands -
Jubeir said Syria's opposition "can't go into talks empty-handed".
HNC leader Riad Hijab said Friday conditions were not yet right for talks to resume, stressing shortfalls in humanitarian aid and breaches of the ceasefire implemented a week ago.
The opposition has demanded the release of prisoners and the delivery of humanitarian aid according to UN Security Council resolution 2254.
De Mistura on Friday said aid had reached 115,000 people in besieged areas.
"Despite the drop in military operations in the field, there has been a pickup in hostilities from the Syrian regime and its allies, including Russia," Makhos said.
Russia's defence ministry reported nine violations of the truce on Saturday, compared with 27 the previous day.
The ceasefire has given some respite to ordinary Syrians, exhausted after five years of war, destruction and shortages.
On Saturday, the day after water returned to pumping stations in Aleppo after a three-month shortage, electricity also slowly returned to Syria's former economic powerhouse.
But attacks continued, including on territory held by the Islamic State group and Syria's Al-Qaeda affiliate Al-Nusra Front, both of which are not included in the ceasefire.
On the political front, the Istanbul-based opposition Syrian National Coalition meanwhile elected Anas al-Abde as its new leader to succeed Khaled Khoja.