"If the negotiations succeed, then we are with the negotiations," rebel spokesman Osama Abu Zeid told AFP.
"If they don't succeed, unfortunately we'll have no choice but to continue fighting."
The comment comes after the rebels refused to take part in face-to-face negotiations in a first session of talks with the regime.
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It was unclear whether the rebels could take part in direct negotiations with the regime at a later session in Astana.
The opening session of the Astana talks could have marked the first time the armed rebel groups engaged in direct negotiations with Assad's regime since the conflict erupted in 2011.
Rebels have said that the talks -- organised by regime backers Iran and Russia and rebel ally Turkey -- are meant to bolster a frail truce brokered by Moscow and Ankara last month.
The regime meanwhile has pushed for a "comprehensive" political solution to the conflict, with Assad insisting that rebels lay down their arms in exchange for an amnesty deal.
More than 310,000 people have been killed and more than half of Syria's population displaced since the start of the nearly six-year conflict.