The head of the key High Negotiations Committee (HNC) opposition umbrella group said in statement that it would agree to a temporary truce only if regime backers halted fire.
Riad Hijab said any ceasefire must be reached "with international mediation and with guarantees obliging Russia, Iran and their sectarian militias and mercenaries to stop fighting". The statement said the opposition "wants to respond positively to
international efforts to stop the Syrian bloodbath".
"But… there will not be a truce unless fighting stops simultaneously on the part of all the belligerents, sieges are lifted, humanitarian aid is delivered to those in need, and prisoners, particularly women and children, are released."
But on the ground, there is little sign that the parties to the increasingly complicated conflict are preparing to halt operations.
Turkey again shelled Kurdish-led forces on Saturday, a day after the UN Security Council rejected a Russian bid to halt Turkish military action in Syria.
Moscow expressed "regret" that the resolution had been rejected, and said it was "concerned at the growing tension at the Syrian-Turkish border".
Russia pledged Saturday to continue backing Syrian government forces against "terrorism", as a key opposition group said
it would support a proposed truce only if regime supporters ceased fire.
World powers have been pushing for a so-called cessation of hostilities to pave the way for renewed negotiations to end Syria's war nearly five years after it began.
But the truce has proved elusive so far, failing to emerge by Friday as originally proposed, even as fighting intensified on the ground amid rising tensions between Russia and opposition backer Turkey.
Russia said UN-led talks on the ceasefire scheduled for Saturday had been postponed to an unspecified later date, raising fresh concerns about whether the truce could be implemented.
In a statement made on Saturday, Syrian President Bashar Al-Assad said that he would accept a ceasefire only if the "terrorists", as he referred to the rebel groups, would not take advantage of the lull in fighting to rearm and if foreign powers such as Turkey and the west cease in supplying the rebels with weapons for the duration of the ceasefire.
AFP contributed to this report.