"A third round without talking about transition would be a waste of time," opposition spokesman Louay Safi told reporters in Geneva, after a week of negotiations ended with no date set for new talks.
"We want to be assured that the regime wants a political solution, not a delaying tactic," he said.
The goal of the talks -- which in January's first round saw the Syrian foes sit down for the first time since the war erupted in 2011 -- is to implement the so-called Geneva communique.
But the plan was never implemented amid spiralling fighting on the ground and a gulf between the opposition and regime over whether President Bashar al-Assad and his acolytes could play any role in creating a future Syria.
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"We're not here to negotiate the Geneva communique but to implement it. Implementation starts with the transitional governing body," Safi said.
The regime insists that before politics the talks must resolve "terrorism" -- its blanket term for a revolt it says is fuelled by foreign jihadists and Gulf money.
"They want to talk about terrorism, but terrorism started with the violent actions of the regime," said Safi.
UN mediator Lakhdar Brahimi said today he was "very, very sorry" that the rounds had ended without progress towards ending the three-year conflict that has claimed more than 136,000 lives.
The regime, he said, had refused to discuss the issue of political change before completely wrapping up the subject of "terrorism".
"We have been disappointed not only by the regime but other sponsors," he said, insisting that "so far clearly the Russians have not prevailed over the regime that wants to stall."
"I'm very sorry to say ... There is nothing positive we can take from these two rounds.